Synthesis of Diamonds and Their Identification

Since 1797, when Tennant demonstrated that diamond consists solely of elemental carbon by comparing the volume of carbon dioxide formed by burning identical weights of charcoal and diamond (Tennant 1797), scientists have been trying to synthesize diamond by converting various carbon-containing substances. Several attempts followed, but it would take over 150 years of continued research before the first successful report of diamond growth was published by scientists at General Electric (Bundy et al. 1955). Diamond synthesis by Union Carbide in 1952 and Allmänna Svenska Elektriska Aktiebolaget (ASEA) in 1953 predate the work at General Electric, but were not reported until later (Liander and Lundblad 1960; Eversole 1962; Angus 2014). The most familiar carbon allotropes are the crystalline phases of graphite and diamond. The hexagonal form of graphite consists of sheets of sp² hybridized carbon atoms in a hexagonal array, with each atom bonded to three equidistant nearest neighbor atoms. The layers are attracted to each other by weak van der Waals forces and may be arranged in a hexagonal, or rarely rhombohedral, stacking sequence. Meanwhile, the carbon orbitals in diamond are sp³ hybridized, with each atom covalently bonded to four nearest neighbors in a tetrahedral arrangement. The prevalent diamond structure is cubic, though the hexagonal lonsdaleite form also exists. Figure 1 illustrates the pressure and temperature (P, T) phase and transition diagram for pure carbon. Theoretically and experimentally determined conditional phase boundary lines separate the diamond and graphite stability fields. The high cohesive and activation energies associated with the different carbon phases mean that other metastable forms can occur under conditions at which they are not thermodynamically stable. For instance, diamond exists at room temperatures and pressures, whereas graphite can survive pressures well into the diamond stability field (Bundy 1980; Bundy et al. 1996).

High quality single crystal diamonds can be synthesized using either the high pressure, high temperature (HPHT) or chemical vapor deposition (CVD) techniques. In the former, growers use an HPHT press to crystallize diamond from a carbon source such as graphite either by direct or indirect (catalytic) conversion methods (Fig. 2), applying temperatures and pressures at which diamond is the dominant phase. The latter technique is instead based on gas-phase chemical reactions involving hydrocarbon and hydrogen gases to deposit metastable diamond at low pressure in a CVD reaction chamber (Fig. 3). Though thermodynamically unstable, under the selected conditions diamond growth is kinetically favorable. This chapter reviews the principles, technology and current capabilities of these two different techniques, as well as the properties of the diamonds that they yield.

Although diamond’s fame originates from its use as an attractive gemstone, research on the synthesis and characterization of their high laboratory-grown counterparts has historically been driven by a scientific community eager to manufacture materials specifically tailored to harness some of the remarkable combination of properties that this material or its lattice defects boast. Industrial and technological applications include diamond abrasives and cutting tools, quantum sensing, high power electronics, thermal management, optical windows, x-ray optics, and electrochemical devices for water treatment (Wort and Balmer 2008; Balmer et al. 2009; Field 2012; Shvyd’ko et al. 2017; Awschalom et al. 2018; Cobb et al. 2018; Koizumi et al. 2018; Donato et al. 2019; Yang et al. 2019; Achard et al. 2020). In the last couple of decades, laboratory-grown diamond quality improved and started to obtain gemological properties akin to natural diamonds, sparking interest in the gem and jewelry industry. Though long considered a niche product with limited distribution, in 2021 laboratory-grown diamonds were reported to account for 5 to 8% of the total global diamond (natural + laboratory-grown) jewelry market; this value was under 1.5% as recently as 2018 (Zimnisky 2021; Branstrator 2022). Market analysis estimated the annual production for laboratory-grown diamonds created for gem applications to be 6–7 million carats (1 carat = 1 ct = 200 mg) in 2020 (Linde et al. 2021). For reference, approximately 40% of the 140 million carats of natural rough diamonds mined annually are gem-quality (Zimnisky 2020). Competition between growers in this sector has led to dramatic rise in diamond sizes, producing faceted gems up to ~15 ct (e.g., Poon and Wang 2016; Wang et al. 2022). Further size increases may be tempered by consumer demand, though growers have been developing methods to synthesize 100 ct crystals. Despite global focus being placed on >1 ct faceted laboratory-grown diamond manufacturing, China has become a significant mass producer of melee-sized goods. Melee diamonds are loosely defined as those weighing 0.001–0.2 ct, but this may vary by country.

The introduction of laboratory-grown diamonds in the gem and jewelry industry has also led to concern that they may infiltrate the natural diamond stream and be sold without full disclosure of their man-made origin. In order to maintain transparency and protect gem and jewelry buyers, scientists and gemologists at grading laboratories have worked in tandem with the diamond physics and geology communities, as well as diamond growers from across the globe, to develop identification methods. This chapter presents a comprehensive account of the key gemological and spectroscopic characteristics that are used to confidently separate natural and laboratory-grown diamonds.

This section discusses the growth of high pressure, high temperature (HPHT) diamonds, synthesized at pressure and temperature conditions where diamond is the thermodynamically stable form of carbon. Commercial HPHT synthesis is generally conducted at higher temperatures and pressure with respect to the graphite–diamond boundary line (see Fig. 1), accounting for kinetic effects related to uncertainty in the boundary line position which may negatively affect the crystallization of diamond from the graphite phase, leading to growth plane incompleteness or inclusion entrapment.

Research teams at General Electric investigated two primary routes for HPHT diamond growth in the 1950s through 1970s: the direct and indirect (stimulated) conversion of graphite to diamond as presented in Figure 4 (Bundy et al. 1955; Bovenkerk et al. 1959; Bundy 1962, 1963; Wentorf 1971; Strong 1989). These approaches differ in technological complexity. Although the former growth method is faster, it requires specialized high-pressure control equipment that can generate and maintain pressures higher than 13 GPa at temperatures above 2000 °C, as well as technical staff capable of manipulating these parameters simultaneously. Thus direct graphite to diamond conversion tends to yield small diamond crystals ranging from a few nanometers to millimeters in size, which may be polycrystalline in nature (Bovenkerk et al. 1959; Bundy 1962, 1963; Irifune et al. 2003, 2004, 2014). Commercial diamond production is based on the indirect graphite to diamond conversion approach, where a carbon source material, usually graphite powder, is mixed with a solvent/catalyst and placed in a growth cell (the “reaction volume”). The solvent/catalyst is used to stimulate growth, transport carbon to the growing surface, and decrease the pressure and temperature requirements for diamond nucleation, with typical conditions of 5–6 GPa and 1300–1600 °C. The reaction volume may also contain diamond seed crystals to aid in nucleation. Diamond grit and powder production can occur with or without diamond seeds, whereas large diamond crystal production requires them. Generally, diamonds grown by the spontaneous nucleation method (no seeds) readily incorporate nitrogen impurities, resulting in yellow color (Chrenko et al. 1971), but specialized approaches can result in colorless diamonds with good morphology, as shown in Figure 5. Color-producing impurities in HPHT diamonds will be discussed in the section Identification of HPHT diamonds: Absorption spectroscopy and color. The temperature gradient catalytic growth method has been successfully used to synthesize multi-carat sized HPHT diamonds (Burns et al. 1999; Kanda 2000; Sumiya and Tamasaku 2012; Sumiya 2014; Sumiya et al. 2015; D’Haenens-Johansson et al. 2015a). Operating under pressures within the diamond stability field, the temperature of the reaction volume is increased so that the solvent/catalyst melts. A temperature gradient of ~20–50 °C is produced, resulting in a hotter environment for the carbon source than the seed region. The carbon source material dissolves in the melt, with the carbon atoms being transported toward the cooler seeds. Under the selected conditions the carbon becomes supersaturated in the melt, crystallizing as HPHT diamond on the seeds. Figure 2 illustrates the key components of this method. HPHT growth occurs along different crystal directions with differing growth rates, typically resulting in crystals with well-developed cube {100} and octahedral {111} faces (cuboctahedral morphology) as shown in Figure 6 (Strong and Chrenko 1971; Kanda et al. 1980, 1989; Sunagawa 1984, 1995; Shigley et al. 1986, 1987, 1992, 1993a,b; Burns et al. 1999). Presently metal solvent/catalysts consisting of pure or alloy forms of Fe, Ni, Co, Mn, Cr, Ta, or Nb are used for industrial and gem diamond synthesis (see the upcoming section Selection of metal solvent/catalyst). Diamond crystallization using alternative non-metallic growth media such as carbonate-silicate, sulfide-carbon, sulfur-carbon, carbon-oxygen-hydrogen, and phosphorus-carbon systems have been reported, with experiments primarily geared toward gaining a better understanding of fundamental diamond formation processes in the mantle or creating unusually doped specimens. Such synthesis falls outside the scope of this chapter, as they are not used for large crystal diamond growth, however Palyanov et al. (2015a) present a detailed review.

High pressure equipment capable of reaching pressures greater than 5 GPa—the minimum necessary for HPHT diamond nucleation—have been developed by several research groups across the world using different approaches for pressure generation and transmission. Excluding rare multi-stage high pressure machines with small (<1 mm³) reaction volumes, the designs can be grouped into four main types: the belt, toroid, BARS, and cubic presses. Figure 7 illustrates the relationship between investment costs, reaction volumes, maintenance costs, and difficulty of usage for these presses. The belt and toroid presses are based on axial compression of the reaction cell, differing in side support design. With the former, a pair of pistons push into the chamber and are supported by binding rings forming a “belt” around the structure (Hall 1960a,b). In the latter, toroidal sub-cavities hold the pressure inside the main semi-spherical cavity (Kolchin et al. 1981; Khvostantsev et al. 2004). In contrast, the BARS and cubic systems apply pressure along multiple axes. BARS presses (referred to by the Russian abbreviation for their description) have a split-sphere multi-anvil block design, using hydraulic oil to generate pressure through an elastic membrane usually made from rubber or similar material (Shigley et al. 1993a; Palyanov et al. 2017). Cubic presses (e.g., Fig. 2) are based on six perpendicular anvils coupled to independent hydraulic cylinders compressing a cubic chamber (Zeitlin et al. 1961). The compression mechanism largely defines the weight and size of the presses. To generate the equivalent load on a similarly sized sample chamber, the mechanical piston hydraulics used for belt and toroid systems are approximately 2–3 times larger than those for BARS systems.

HPHT presses are manufactured globally, with certain regions specializing in a particular type. Factories in former USSR countries produce the most popular series of BARS presses, with the model names (300, 350, and 400) describing the diameter of the sphere in millimeters. BARS presses use an oxide/halide ceramic cube (or parallelepiped) high pressure cell (HPC), where the edge defines the reaction volume, e.g., a 32 mm cube contains a 2 cm³ reaction volume. These presses can produce loads of 6–12 MN (Feigelson 1983). Toroid systems also originate from this region, with central recess (lune) diameters of 20, 30 and 40 mm in combination with hydraulic press models DO043 or DO044. These presses are capable of generating 20 and 25 MN loads, respectively. The diameter of the lune defines the reaction volume, e.g., a 40 mm diameter provides an approximately 2 cm³ reaction volume. Reaction volumes up to 10 cm³ have been reported (Palyanov et al. 2015a). Belt systems from the United States are used for diamond grit production, with reaction volume sizes up to 1000 cm³. The Chinese development of cubic systems has benefited from government support, leading to a continuous improvement in reaction volume sizes, ranging from the series 460 to the series 1000 (as of June 2020). The series number represent the diameter of the hydraulic cylinder in millimeters. The series 560, 650 and 750 cubic presses are the most widely used models for both grit and large diamond growth, attaining loads of 30–50 MN. As for BARS presses, the cubic HPC edge defines the reaction volume, e.g., 74 mm cubes have reaction volumes of about 40 cm³.

All four press types consist of three main parts: the pressure generating machine, the pressure transmitting unit (referred to as the high pressure apparatus, HPA), and the sample containing HPC. In order to generate and hold the pressure on the sample, the force is transmitted from the press machine through the respective HPAs to the HPCs (Figs. 2 and 8). The HPA is made from cemented carbides composed of tungsten carbide and cobalt (WCCo alloys, such as WC-6% Co, WC-8% Co and WC-10% Co). The geometry of the HPC is dictated by the shape of the compressive and tensile components of the HPA. Presently, the hardness and toughness of the HPA hard alloy set the upper limit for the achievable pressure in systems used for large volume diamond mass production. For further details regarding HPA and HPC design see Bundy (1988). Future improvements in large volume superhard HPAs would propel the diamond industry to increase production volumes, improve sample quality, and further explore promising unconventional growth media for diamond synthesis.

Heavy investment in diamond synthesis using cubic presses have led to design upgrades and regular breakthroughs in diamond volumes and qualities, warranting further detailed description of this press type. Cubic high pressure cells (Fig. 2b and 8b) are made of powder pressed pyrophyllite with a central cavity containing tens of concentric and axially assembled components designed for three main functions:

1. Pressure transmission to the sample reaction chamber;

2. Resistive heat generation to melt the metal solvent/catalyst;

3. Providing thermal and chemical insulation to prevent damage to other press components during high pressure and temperature experiments over long periods of time.

These functions are generally applicable to all HPHT systems. Components are made of advanced materials in order to fulfill these functions and withstand the combination of extreme pressures (5–7 GPa) and temperatures (1300–1800 °C) produced in the sample reaction chamber. Certain salts and refractory ceramics possess the required chemical inertness, high melting temperatures, and phase stability at high pressures. Boride and chloride salts, as well as ceramics in the form of oxides (Mg, Al, Zr, or Y) are commonly used by the diamond synthesis industry (Strong 1977; Zhu et al. 2019). The component materials may differ for systems that require higher temperatures, such as those used for HPHT annealing treatments to change diamond color.

The temperature in the cell is generated by means of graphite or composite material resistive heaters. A precise temperature field distribution is formed in the diamond synthesis region by careful selection of the geometry and physical properties of the electric system, considering how these may be affected when subjected to simultaneous temperature and pressure changes. Temperature gradients across the cell are undesirable for diamond grit production, with efforts made to minimize fluctuations close to the cell sides. In contrast, controlled axial and radial temperature gradients are the basis of the temperature gradient growth method used for large high quality diamond synthesis (e.g., D’Haenens-Johansson et al. 2015a). Careful component designs which consider detailed electric and heat conductance properties in the cell enable control of the heat generation and dissipation distributions, offering the flexibility to grow samples using either scheme. Finite element method (FEM) simulations are widely used to characterize the temperature field in the growth cell under different conditions (Li et al. 2013). The main drawback with this approach is that it cannot consider the component material properties at high temperatures and pressures. Results from complementary experiments analyzing real temperature distributions using thermocouples, as well as observations of phase transitions, provide the necessary feedback to improve the simulated material properties. Temperature gradients as fine as 1 °C/mm can be predicted and realized within the cells, facilitating the set-up of the optimal carbon supersaturation conditions necessary for growth of high quality diamond crystals with flat faces and good morphology. In the case of suboptimal carbon supersaturation, growth is unstable, resulting in the formation of multi-crystal aggregates known as druzes (Fig. 9).

Historically, HPHT diamond growers focused on selecting metal solvent/catalysts with the lowest possible diamond nucleation temperatures due to practical and economic reasons. Lower growth temperatures in turn also meant that lower pressures could be used, easing the demands on the press and increasing the lifespan of the HPA components. The costs of the metals used were also considered. Initially, the solvent/catalysts were Mn and Ni alloys (Bundy 1963). However, there was a great disadvantage to their use: the growing crystals readily trapped inclusions of carbides associated with these metals. Thus diamond crystal powders produced at the time were characterized by poor toughness. Inclusion uptake could be decreased, and the material properties improved, by operating at higher temperatures and pressures using Fe, Ni and Co alloys. Growth using a variety of solvent/catalysts composed of combinations of invar (Fe–Ni) and kovar (Fe–Ni–Co) alloys have been carefully analyzed (Strong and Chrenko 1971; Kocherzhinski and Kulik 1996; Sugano et al. 1996). Eutectic compositions are used as they lower the melting temperature.

Diamond growers devise proprietary melts with additional elements to decrease the pressure and temperature parameters or improve the crystal characteristics. Such additives may either be elements which are fairly chemically inert with respect to carbon (Cu, Sn, In, As, Zn, or Au), or they may be known to interact, forming carbides (Cr, Si, V, Mg, Nb, or Zr). These latter carbide formers are also strong nitride formers, and are usually added specifically to getter nitrogen that would otherwise readily incorporate into the growing diamond. As the molten solvent/catalyst should contain more than 10 at.% carbon, additives may be used to increase carbon solubility (Strong and Chrenko 1971; Kanda 2000). The resulting complex metallic mixtures have been tested for diamond production at a range of pressure and temperature conditions to optimize crystal quality. The conclusions are generally that growth benefits from higher temperature and pressure parameters. Intentional or inadvertent doping of the diamond crystals due to impurities in the melt ingredients, carbon source, or cell components further complicate growth and expand the variables that growers can customize. These dopants, further discussed in the section Control of color center content and extended defects, affect the HPHT diamond properties, most notably color. Large, high quality HPHT diamonds that are as-grown colorless, blue, green, and yellow can be produced.

The metal solvent/catalyst is one of the factors that affect the morphology of the growing crystal. For instance, Kanda et al. (1989) demonstrated that pure Ni melts resulted in cubic {100} and octahedral {111} faces, whereas Ni-alloys which included Co and Fe promoted the development of dodecahedral {110} and trapezohedral {113} faces. The solvent/catalyst composition also influences surface structures on the crystal faces, which may form dendritic, vein-like, lamellar or etch pit patterns (Kanda et al. 1980). The patterns correspond to the solvent/metal textures that are created during the quenching process that occur at the end of growth runs.

Large HPHT diamond growth using the temperature gradient method is initiated homoepitaxially on a diamond seed. Growth on the seed can follow two different approaches. Either all sides of the seed can be overgrown with HPHT diamond (like natural diamond growth), or growth can be restricted to one face, with greater control over the orientation. Figure 10 shows diamonds grown using the first method, with a yellow diamond seed enveloped by new colorless diamond material. For the second method the selected growth face of the seed is placed in contact with the metal solvent/catalyst, while the other faces are shielded by some barrier material (Wentorf 1971). The latter is the most commonly used approach for HPHT diamond growth.

The maximum size of each HPHT diamond is defined by the number of seeds simultaneously used as the growing diamonds compete for a limited amount of carbon source material and space within the reaction cell. It may be possible to increase the crystal output if the reaction cell is large enough to be segregated into a multi-layered growth structure. An example of multiple seeds for a two-layered growth cycle is shown in Figure 11. During growth the layers were vertically adjacent to each other, and advanced techniques were used to generate the necessary temperature gradient across the system to initiate diamond nucleation and growth under the appropriate pressure and temperature conditions.

The orientation of the diamond nucleation face on the seed defines the crystallographic orientation of the first atomic-scale HPHT overgrowth layer. The growth directions of subsequent atomic layers are influenced by the pressure and temperature parameters as well as the chemical composition of the growth environment, usually resulting in the characteristic cuboctahedral morphology (Strong and Chrenko 1971; Kanda et al. 1980, 1989; Burns et al. 1999). Typically, the upper growth face of the HPHT diamond has the same orientation as the nucleation face on the seed, unless the sample has experienced significant variations in growth conditions. Consequently, growers are able to exercise some control over the growing crystal morphology. Figures 6, 9, 11, and 12 present examples of HPHT diamonds grown on (100)-oriented (cubic) seeds, while Figure 13 shows samples grown on (111)-oriented (octahedral) seeds. Although intentional growth on alternative seed facet orientations such as (113), (115), or (110) are also possible, in practice it is mainly restricted to fundamental growth research as seed preparation is challenging and availability is limited.

Seed size selection has to balance material costs (larger seeds are more expensive) and the grower’s ability to initiate and control growth without multiple nucleation sites. Seeds range from tens of micrometers to a few millimeters in size. There is a plentiful supply from the industrial diamond grit and powder manufacturing sector.

Growth temperature affects the crystal morphology, with the primary faces changing from {100} to {100} + {111} and then {111} as temperatures are increased when using standard ferrous solvent/catalysts (Satoh et al. 1990; Sumiya et al. 2002). Temperature ranges for good crystal quality are narrow, with reported windows spanning only about 10–40 °C, depending on the applied pressure and the type of diamond being synthesized. At lower temperatures skeletal crystals start to form, where growth primarily proceeds along corners and edges rather than at the crystal faces. At higher temperatures inclusion uptake increases. However, there are exceptions to this behavior. Rare pure rhombidodecohedral {110} (12-faced crystal form of {110} with cubic crystal symmetry) diamonds can be grown under certain pressure and temperature conditions for a given metal solvent/catalysts, as shown in Figure 14. The relationship between the pressure, temperature, solvent/catalyst, and resulting grown diamond morphology is complex. For example, the HPHT diamonds shown in Figure 10 were grown at high temperatures (>1850 °C), yet are dominated by {100} faces. These diamonds were grown using an atypical Mg-based solvent/catalyst.

The solubility of impurities such as nitrogen into the solvent/catalyst rises with temperature, potentially leading to lower impurity concentrations in the growing diamond (Burns et al. 1999). Furthermore, increasing temperatures leads to a decrease in the efficiency of impurity getters (Kanda and Lawson 1995). Hence drifting or fluctuating local temperatures at the growth fronts can modify the distribution of impurities within growth sectors, defined as volumes within a diamond that were grown with common growth planes (Kanda and Yamaoka 1993; Burns et al. 1999). Further discussion of growth sectors is presented in the section Identification of HPHT diamonds: Growth sectors. Higher temperatures in nitrogen-containing growth can also lead to the formation of aggregated nitrogen defects (nearest neighbor nitrogen-pairs known as A-centers) (Davies 1976; Kanda et al. 1990; Vins et al. 1991; Babich et al. 2000, 2004).

Pressure influences significantly the nucleation rate in the indirect graphite to diamond conversion growth method (Bundy et al. 1955), whereas for the temperature gradient method its influence is relatively small and restricted to the effect of “widening” the effective technological growth zone. It has been observed that pressure has some influence on the solubility of carbon in melted solvent/catalysts, so that fluctuations also affect the crystal growth.

While diamond nucleation and/or seeded growth have been achieved using many pressure and temperature combinations, temperatures exceeding 1600 °C and pressures above 7 GPa have been identified as suitable for stable mass production of diamonds. However presently such parameters are not economically viable to generate in larger volume high pressure systems since production costs would be much higher than the sale price of the product, without much improvement in the diamond properties.

Diamond is a wide-bandgap semiconductor with a bandgap energy of ~5.48 eV, making it transparent from the ultraviolet, through the visible, and into the infrared range (~225 nm–2.6 μm) (Clark 1959; Collins 1982). Consequently, perfect, impurity-free diamond consisting solely of carbon atoms is a colorless material. The growth environment, as well as post-growth conditions or treatments may however introduce color-producing impurities and defect complexes. Diamond has one of the highest atomic densities (atoms/cm³) of any terrestrial material. This is due to the small size of carbon atoms and the four strong and short carbon to carbon sp³ bonds per each carbon atom in the lattice. As a result, it is difficult to fit larger atoms into the lattice. Nitrogen, boron, and possibly phosphorus atoms are known to occupy lattice sites, while larger atoms such as silicon and nickel usually are accompanied by vacancies for them to fit in the lattice. Defects related to carbon interstitials are observed and there is evidence for atomic hydrogen and protons associated with elemental impurities. Defects can change the electrical, optical, thermal, and structural properties of diamonds. Thus the ability to manipulate the defect content in laboratory-grown diamonds through doping or growth control measures are important. Many defect centers absorb light, fluoresce, luminesce, and/or phosphoresce, enabling their identification, as discussed in detail in the section Separation of natural and laboratory-grown diamonds. Collins (1982, 1992, 1993a), Ashfold et al. (2020), and Green et al. (2022, this volume) present reviews on diamond defects.

Nitrogen and boron are the most widespread impurities in both natural and laboratory-grown diamonds (Collins 1993a,b; Ashfold et al. 2020). Unintentional contamination with them during HPHT synthesis is common due to their availability in the growth environment. Air, which has 78.09% nitrogen by volume, is easily trapped in pores and cavities within the reaction cell or its components. Trace amounts of both nitrogen and boron can be found in the carbon source, metal solvent/catalyst, and cell materials. Whereas the purity of the solvents and graphite can be controlled with high accuracy (up to 0.001 wt%), the purification process of the total assembled growth capsule is highly complicated and not reproducible due to constraints in press operation. For these reasons, precise control of nitrogen and boron content in the grown diamonds primarily relies on a chemical approach—the use of impurity getters (Mg, Al, Zr, Ti, V, Cr) or compensators (B, S, N, P). Getters act as chemical absorbers of the undesirable elemental impurity, for instance by forming nitrides, minimizing the element’s availability for dissolution into the diamond lattice (Strong and Chrenko 1971; Wentorf 1971; Sumiya and Satoh 1996; Burns et al. 1999; Wakatsuki et al. 1999). Rather than preventing impurity incorporation into the growing crystal, compensators form lattice defects that interact with those formed by impurities, donating or accepting electrons and reducing the concentration of a specific defect charge state which produces unwanted properties (e.g., color) (Burns et al. 1990).

Figure 12 shows the effect of careful addition of getters to control the concentration of nitrogen in HPHT diamonds. As the getter content is decreased, there is an increase in the concentration of isolated nitrogen defects and a transition from Type IIa, to IIa + Ib (where the crystal contains both Type IIa and Ib material), to Ib (see Identification tools section below for an explanation of diamond types). There is a stark change in the appearance of the diamonds as the isolated nitrogen defect is associated with yellow color (Chrenko et al. 1971), as discussed in Identification of HPHT diamonds: Absorption spectroscopy and color. This control opens up the possibility of defect engineering, where specific concentrations of desirable color centers are produced, either in “as-grown” (untreated) material, or through treatment. Nitrogen-vacancy (NV) centers, which can produce an attractive pink color in gem quality diamonds (e.g., Fig. 15), are also of great interest to the scientific community as they develop methods to harness properties of the negatively charged state of the center (NV^–) for uses in quantum technology and other fields. NV centers can be created by irradiating and annealing HPHT diamonds with suitable isolated nitrogen concentrations (Moses et al. 1993; Shigley et al. 2004a). The addition of nitrogen getters narrows the range of temperatures at which diamonds with good crystallinity can be grown, and leading to higher uptake of melt inclusions and associated carbides (Sumiya and Satoh 1996; Burns et al. 1999, 2009; Sumiya et al. 2002). It is thought that nitrogen getters decrease the solubility of carbon in the metal solvent/catalyst, or prevent the diffusion of carbon. Group Ib metals (Cu, Ag, Au, etc.) can be used to decompose the carbides (Wakatsuki 1966). Nitrogen getters can also affect the crystal morphology (D’Haenens-Johansson et al. 2015a).

Uncompensated boron gives rise to blue color in HPHT diamond (Collins and Williams 1971) (see also Identification of HPHT diamonds: Absorption spectroscopy and color). Impurity uptake in HPHT diamonds is sensitive to the orientation of the growing face, growth rates and temperature (Strong and Chrenko 1971; Burns et al. 1990, 1999; Satoh et al. 1990). Boron concentrations are typically higher in {111} growth sectors, followed by {110} sectors, whereas nitrogen concentrations follow {111} > {100} > {113} > {110} (Burns et al. 1990). Even blue color can only be achieved by reducing nitrogen concentrations in the diamond to less than 10¹⁸ cm^–3. Otherwise, the resulting material will contain both yellow Type Ib and blue Type IIb growth sectors, which may produce an overall green hue (Shigley et al. 1992, 2004a,b; Fritsch and Shigley 1993; Rooney et al. 1993; Hainschwang and Katrusha 2003). Both Type IIb or IIa + IIb HPHT diamonds are thus grown by adding a nitrogen getter and a boron dopant (e.g., amorphous boron). An example of sector-dependent boron uptake is shown in Figure 16. Further discussion on this phenomenon is presented in Identification of HPHT diamonds: Growth sectors.

Although nitrogen and boron are the most studied impurities in HPHT diamonds, several other elements such as nickel, cobalt, silicon, and germanium can be introduced during synthesis, forming vacancy-containing defect complexes (Collins and Spear 1982; Collins et al. 1990; Lawson and Kanda 1993a,b; Lawson et al. 1996; Sittas et al. 1996; Palyanov et al. 2015b, 2017). These impurities often originate from the solvent/catalyst and its additives.

The growth of extra-large diamond single crystals with high crystalline quality has been simultaneously a dream and one of the biggest challenges for diamond researchers during the last 60 years. Progress was tempered by the technological expertise required, the high associated costs, as well as underdeveloped applications for the final product. Before around 2010 large diamonds were grown using modified belt presses that include large volume HPAs with precise cooling water, hydraulic pressure suspension and in-cell temperature control. De Beers Diamond Research Laboratory grew a 34.80 ct yellow-brown Type Ib HPHT diamond in 1992 (pictured in Kanda (2005), while Sumitomo Electric Industries synthesized 10 ct colorless Type IIa diamonds (Sumiya and Tamasaku 2012; Sumiya 2014; Sumiya et al. 2015). The team at New Diamond Technology (NDT) Ltd. (headed by the present author ANK) adapted cubic series 750 and 850 presses to grow large diamond crystals, and in 2015 announcing the growth of a 32.26 ct colorless diamond that was later cut into a 10.02 ct E color gem (Deljanin et al. 2015). Continuous improvements in their growth capabilities led to increasing rough HPHT diamond sizes from 1 to 60 ct for a reasonable cost and in short time periods up to about 2 weeks. Previously the generally accepted averaged mass growth rate for good single crystals was 1–2 mg/hour, whereas the NDT team could achieve rates as high as 45 mg/hour in those record stones (Deljanin et al. 2015; D’Haenens-Johansson et al. 2015a). To date NDT has produced the largest faceted gem laboratory-grown diamonds reported, including 20.23 ct Fancy Vivid yellowish orange, 15.32 ct H color, and 10.08 ct Fancy deep blue diamonds (Poon and Wang 2016; Ardon and Eaton-Magaña 2018; Wang et al. 2022). As of the year 2020 at the facilities of Alkor-D Ltd. the author ANK has broken the 100 ct limit of single crystal rough HPHT diamonds, as shown in Figure 17. They have also produced a Guinness World Record-breaking 150.42 ct blue HPHT diamond (28.55 × 28.25 × 22.53 mm), not pictured (IGI 2022). Notably these Type IIa and IIb diamonds were grown using extremely fast average growth rates (>64 mg/hour). The limited reaction volume resulted in some deterioration of the exterior of the samples.

There are three factors that are extremely important when growing extra-large diamond single crystals: temperature control and reproducibility within the given in-cell isotherm pattern, high chemical stability of the hardware materials used to withstand prolonged exposure times (up to 400 hours), and the availability of a large growth volume for the solvent bath and the necessary quantity of carbon source. It is expected that increasing the maximum achievable cubic press load up to 80–90 MN and increasing the high pressure cubic cells to span 90–100 mm will make it possible to grow diamonds weighing as much as 500 ct (100 g). In addition to attracting interest from the gem industry, future applications include manufacturing single crystal diamond plates with pre-determined characteristics (e.g., high crystalline purity, charge carrier mobility, thermal conductivity or electrical conductivity), with uses as varied as substrates for CVD growth or diamond x-ray optics components (Stoupin et al. 2016; Shvyd’ko et al. 2017).

This section discusses the growth of diamond by chemical vapor deposition (CVD) under conditions where diamond is not the thermodynamically stable form of carbon, as highlighted in Figure 1. The earliest reports of verifiable diamond growth under metastable conditions (low pressure) date to the 1950s to 1970s, initially in the United States, Russia, and then Japan (Angus 1994; Fedoseev 1994; Setaka 1994). A detailed summary of the history of chemical vapor deposition of diamond is provided by Angus (2014). The inclusion of hydrogen as a key reactant/solvent initiated the use of hot filaments and plasmas for creating atomic hydrogen and activating the reactants which demonstrated growth of mm-sized crystals. The mid 1980s then saw an explosion of interest in CVD diamond research forming the basis of our current understanding and diverse applications of diamond materials by groups primarily in Japan, US, and Europe (Angus 2014).

Crystalline diamond can be grown by chemical vapor deposition with a multitude of different gaseous reactant activation techniques: hot surfaces (hot filaments), plasmas (DC, RF, microwave, e.g., Fig. 3), and combustion flames. As will be presented later, there are many common chemical processes to these apparently dissimilar techniques. In the hot filament and plasma-based techniques, the reactant gases contain primarily hydrogen and a dilute amount of some hydrocarbon (generally less than 10%). The reactants are activated at pressures between ca. 1 mbar to several bar (typically between 20 and 500 mbar) to create many molecular, radical, and ionic species. Figure 18 presents a schematic of the diamond CVD process. In this schematic, the reactants are introduced into a pressure containment vessel (reactor) and the pressure is maintained with a throttle valve to the exhausting system (pump). The hot filaments (typically tungsten at 2000–2500 °C) or the plasma (gas temperature in the range of 2000–5000 °C) dissociate the reactants and initiate complex gaseous chemical reactions while the gas is transported by forced flow, convection, and/or diffusion toward the growing diamond crystals whose surface temperature is maintained at between 600 and 1200 °C. “Boundary” layers are formed just above the growth surfaces with strong temperature gradients, reactant gradients, and gas stagnation. Growth then proceeds on the surfaces of the diamond crystals. Generally, diamond seed crystals of sizes between several nm to several cm are provided to start the process, commonly referred to as “substrates.” However, nucleation techniques for forming the diamond seeds from stable carbon/carbide clusters on the surface are also used (Liu and Dandy 1995).

CVD diamond materials can be polycrystalline agglomerates with a continuum of grain sizes from nm to cm, or single crystal blocks or plates. Various properties of these materials can also be tuned by controlling the grain size, defects, and impurities (Butler and Windischmann 1998; Butler and Sumant 2008).

The CVD growth of crystalline diamond is one of the most complex of any chemical vapor deposition or physical vapor deposition process (PVD) (Butler and Woodin 1993; Goodwin and Butler 1997; Butler et al. 2009a,b). Its study involves many scientific disciplines: chemical kinetics, plasma chemistry, fluid dynamics, thermal transport, surface chemistry, and possibly, diffusion processes in solids.

The temperature of a gas, surface, or solid is an extremely important parameter for controlling the kinetics of reactions and transport of species. While there are reports of very slow, poor-quality diamond growth by CVD at temperatures as low as ∼300 °C, the optimal surface temperature is generally between 800 and 1200 °C for excellent crystallinity and reasonable growth rates. Surfaces at over 800 °C are sufficiently hot to break even the strongest of single chemical bonds that an adsorbate might make with the surface. Reactants can only bond to the surface with very strong bonds, and even those that can bond are likely to desorb back to the gas after a short time. Hence, the incorporation of species into the growing bulk will likely involve the formation of multiple chemical bonds to the surface, while desorption of reactants to the gas will dominate over the mobility or diffusion of reactants across the surface.

Bulk processes like structural rearrangements or diffusion of atoms are very unlikely at the bulk temperatures of diamond CVD (less than 1200 °C), because of the extremely high “Debye” temperature of bulk diamond, ∼1800 °C. The Debye temperature is a parameter measuring the transition of the heat capacity of a solid from quantum phonon statistics to classical thermodynamics, and as such is a measure of the thermal motion of atoms in the bulk. Heating diamond to approximately its Debye temperature initiates the transition to the more thermodynamically stable form of carbon—graphite—catalyzed at defects and surfaces (Evans 1979). A consequence of this is that there is very little thermal motion in the diamond lattice during growth, with diffusion possible only for lattice vacancies and interstitials, and with little repair of structural defects.

The nucleation of diamond on non-diamond substrates can happen, but is a poorly understood process. Important factors include a source of carbon to form clusters of carbon on or within the surface, bonding of the carbon cluster to the substrate to prevent desorption at the surface temperatures, and etching/rearrangement reactions within the cluster to convert to local sp³ diamond structure. See Pehrsson et al. (1993) and Liu and Dandy (1995) for more detailed discussions. For the most part, current researchers and growers of CVD diamond who grow on non-diamond substrates will prepare the substrate for growth by treating or seeding the surface with crystalline diamond particles (Clausing et al. 1989; Inspektor et al. 1989; Philip et al. 2003; Buijnsters et al. 2005). Depending on the application, these particles can range in size from nm to mm. Then subsequent growth is homoepitaxial growth of diamond on diamond.

The important insight enabling the growth of diamond at low pressure was the recognition of the role of hydrogen in stabilizing the surfaces of diamond and in etching non-diamond carbon. This is discussed at length in the review of the early history by Angus (2014). One major advantage of CVD diamond growth over HPHT techniques, or diamond growth in the mantle of the earth, is that scientists can employ a range of in situ analytical techniques to study the process. The first in situ examination of the gas phase composition during diamond CVD with infrared absorption spectroscopy detected the conversion of the reactants (methane 0.5% in hydrogen) to C₂H₂ and CH₃ in a hot filament activated deposition chamber (Celii et al. 1988). Direct detection of ground state atomic H in the gas phase of hot filament and plasma based reactors followed (Celii and Butler 1989; Celii et al. 1990a,b). Optical emission from excited states of atomic hydrogen was also observed in plasma activated diamond CVD (Stalder and Sharpless 1990; Chu et al. 1991; Muranaka et al. 1991). Mass spectroscopy sampled in situ from a hot filament reactor detected CH₄, C₂H₂, and C₂H₄ inferred the presence of methyl (CH₃) radical and H atoms in the complex gaseous chemical processes (Harris et al. 1988; Harris and Weiner 1990; Hsu 1991). Sampling of the exhaust gases from a microwave plasma detected CH₄, C₂H₂, and C₂H₄; and CO and H₂O when O₂ was added to the reactant gases (Johnson et al. 1990; Hsu et al. 1994).

Which chemical species are important for the CVD growth of diamond? The conclusion reached by many experiments is atomic H and the CH₃ radical. Kinetic and isotopic experiments support that the bulk of the carbon incorporated into the diamond crystal is from CH₃. For an extensive review, see Goodwin and Butler (1997). The typical growth environment involves a gas pressure above 30 mbar and a relatively long gas residence time in the reactor, allowing for the complex gaseous kinetics to approach a stable equilibrium. An approximate model of the observed gas phase composition was presented and is shown in Figure 19 (Butler et al. 1989). In this model, the detailed composition of the gas phase can be approximated by the thermodynamic composition of the input reactants placed in a container at an equilibrium temperature of the plasma or hot filament source. An important consequence of the model is the implication that diamond growth is not sensitive to the specific nature of the hydrocarbon reactant, but just to the ratio of carbon to hydrogen in the reactants. While this “zero order” approximation is useful in a general sense, the gaseous chemistry is far more complex than this due to the spatial and temperature gradients in the growth environment.

The interconversion of the dilute hydrocarbon species in atomic and molecular hydrogen is shown schematically in Figure 20. In the hotter portions of the gas (in the plasma), the chemistry is driven toward the lower right of the figure, by the conversion of CH_n to C₂H_n, while in the colder portions of the reactor, near the reactor walls and the growing surface, the chemistry is driven to the upper left by atomic hydrogen reactions (C₂H_n to CH_n) (Ma et al. 2009).

The composition of the gas near the hot filaments or in the plasma (>2400 °C) is very different than the flux of gaseous species colliding with the growing diamond surface. This is because the stagnant gas at or near the growing surface has (nearly) the same temperature as the growing surface (800–1200 °C). Thermal and compositional boundary layers form with sharp gradients between the bulk gas and the surface.

Since diamond growth occurs on the surface of the diamond substrates, the critical chemical species causing growth must collide with the surface at a rate at least as fast as carbon is incorporated in diamond. Extensive experimental measurement and kinetic modeling have determined that the only species meeting this criterion in the CVD environment are H₂, H, CH₄, CH₃, and C₂H₂, and that the dominant carbon contributor is the CH₃ radical. Goodwin and Butler (1997) provide a detailed review and discussion.

The surface of the growing diamond is terminated by hydrogen, forming C-H bonds, very strong single bonds with strengths ∼ 96 to 102 kcal/mole (Calvert and Pitts 1966). Temperatures above 850 °C are sufficient to slowly break these strong bonds and desorb hydrogen (Koleske et al. 1994, 1995). In ultra-high vacuum, “clean” non-hydrogenated surfaces can be produced by annealing to over 850 °C (Pate 1986; Thomas et al. 1992), but in a gas containing atomic and molecular hydrogen and hydrocarbon species, the surfaces are partially hydrogenated by the balance of desorption and reactive adsorption. The limitations on the surface temperatures for CVD diamond growth are set by the hydrogen termination of the surface: for complete hydrogen coverage there are no reactive sites, and when there are no terminating hydrogen atoms, the surface reverts to sp² bonding and converts to graphite.

Given the shortness of the C-C bond in diamond, the surfaces have an extremely high areal density of surface carbon atoms (1 to 3 × 10¹⁵ cm⁻²). Termination of all the carbon surface bonds with any non-carbon atom except H(D) is impossible. Even H is too large to terminate all the bonds on the (100) surface. This surface undergoes a (2 × 1) surface reconstruction to form carbon dimer chains on the surface (Hamza et al. 1990). Growth surfaces are not necessarily flat low index planes, but contain many steps and local geometric environments. Figure 21 displays ball and stick models of steps on the (111), (110), and (100) 2 × 1 surfaces.

A “standard mode” of diamond CVD has evolved over the years with many contributors and is summarized in (Butler and Woodin 1993; Goodwin and Butler 1997; Butler et al. 2009a,b). The key elements of the model are:

The diamond lattice is stabilized and prevented from rearrangement to graphitic carbon by termination with hydrogen atoms (or a similar chemical species), while the temperature is too low for spontaneous bulk rearrangement to occur (i.e., below the Debye temperature of diamond)

The gaseous activation process dissociates molecular hydrogen into atomic hydrogen which reacts with the source hydrocarbon to create a complex mixture of hydrocarbon species including reactive carbon-containing radicals. The H atoms created by the gaseous activation process also abstract hydrogen from the surface C-H bonds, thereby creating surface radical sites. These radical sites will occasionally react with gas phase carbon-containing radicals such as methyl radicals (as discussed above), resulting in a strongly bound adsorbed carbon species. Much more frequently, however, the radical sites are simply refilled by recombining with gaseous H atoms. This constant turnover of the surface-terminating species (hydrogen) drives the surface chemistry to dehydrogenate the adsorbed carbon species and to incorporate carbon into the lattice by creating multiple bonds to the lattice

The atomic hydrogen, and, to a lesser extent, other gaseous species, react with any sp or sp² carbon sites on the surface, converting them into sp³ bonded carbon

CVD epitaxial growth of single crystal begins with a single crystal diamond substrate, usually in the form of a thin plate (ca. 200 to 500 μm thick). The surfaces of the plate may be polished, laser cut, or cleaved. The most popular orientation of the upper surface is near or on-axis (100). Other orientations like (110), (111), and (113) are less frequently used. The quality and orientation of the seed crystal may be important for some applications. For example, dislocations in the seed generally propagate in the epitaxial layer, and impurities or dopants tend to incorporate preferentially on certain crystal faces and local structures (Martineau et al. 2004; Gaukroger et al. 2008; Friel et al. 2009; Tallaire et al. 2017a). An example of CVD diamond growth on a yellow Type Ib seed is shown in Figure 22, where the epitaxial growth was performed in two sequential growth runs. When viewed from the growth side to the seed, the color is dominated by that of the seed. Dark polycrystalline diamond rims readily grow on the sample edges, potentially encroaching on the growing diamond and shrinking the top surface, unless precautions are taken to minimize their formation (Mokuno et al. 2005; Nad et al. 2015, 2016). Such blocks are often cored with a laser machining system to remove the outer rim and the initial seed. Figure 23 shows the result after coring. Figures 22 and 23 display a common result, however with the proper recipe and conditions, it is possible to expand the sides of the seed crystal without the polycrystalline rim (e.g., Tallaire et al. 2017b).

Since sizable natural and HPHT-grown crystals are rare or expensive, larger areas of quasi single crystal material are generated either by tiling multiple plates of well oriented single crystals and growing them together (Yamada et al. 2011, 2013, 2014; Tallaire et al. 2013), or by nucleating highly oriented diamonds seeds on uniquely prepared non-diamond substrates and growing them to a thickness where they appear as single crystal material with a high dislocation density (Gallheber et al. 2018).

Crystal twinning occurs in all forms of crystalline diamond (Kohn 1958; Varma 1970; Walmsley and Lang 1983; Angus et al. 1992). It is particularly common to CVD-grown material and material grown at very high growth rates, e.g., HPHT diamonds. It may occur at the pressures and temperatures of natural diamond or HPHT diamond synthesis by atomic rearrangement of the lattice (annealing) to relieve stress due to plastic deformation of the crystal after growth, or as part of the growth of the crystal (Lang 1979; Palyanov et al. 2015a). During growth by CVD where there is little or no motion of the atoms in the lattice, only one model has been proposed for how a twin is formed during growth (Butler and Oleynik 2008). In this model a twin can only form on a local part of the diamond surface with an exposed {111} plane by the building of a three-atom bridge between adjacent surface carbon atoms to form a four-atom island nucleating the next layer of growth. Due to rapid isomerization of the three-atom bridge at the growth temperature, the result is a nearly 50:50 chance that the four-atom island will be twinned relative to the underlying crystal structure.

In the CVD growth of diamond, intentional doping is accomplished by adding volatile compounds of the intended dopant to the gaseous reactants, for example N₂, B₂H₆, SiH₄ or PH₃, or by placing a solid compound containing the intended dopant in the reactor and exposing it to the plasma, such as solid Si or B. Both “p-type” and “n-type” electrical conduction is observed with B and P doping, respectively (Lightowlers and Collins 1976; Koizumi et al. 1998). A dopant of particular note is nitrogen, which is known to have a catalytic effect on the growth rate (Chayahara et al. 2004; Tallaire et al. 2006; Achard et al. 2007; Butler and Oleynik 2008). Nitrogen contamination can occur from gases, leaks or desorption from reactor components, but careful growth can produce high purity CVD diamonds with nitrogen concentrations below 1 ppb (parts per billion carbon atoms, 10¹⁴ cm⁻³), which are attractive for electronic applications (Isberg et al. 2002; Tallaire et al. 2006; Edmonds et al. 2012). Intentional doping using N₂ can result in the incorporation of up to tens of ppm (parts per million) of nitrogen (Edmonds et al. 2012). For the growth of optically transparent material or larger samples, nitrogen-doping can be beneficial, with even low concentrations of nitrogen in the gas phase (~10 ppm) resulting in nearly a tripling of the growth rate (Tallaire et al. 2006). However, a balance must be struck as high concentrations of nitrogen introduce brown color in the material, attributed to the formation of vacancy clusters and/or nitrogen-related defects (Barnes et al. 2006; Jones et al. 2007; Mäki et al. 2007; Khan et al. 2013; Zaitsev et al. 2020). The presence of nitrogen in the growth gases also has a pronounced effect on the growth mode, transitioning from layer-by-layer growth to step-flow with macroscopic step-bunching (Yamada et al. 2016). Unlike natural diamonds, nitrogen in CVD diamonds is found primarily as isolated substitutional nitrogen (N_S), nitrogen-vacancy (NV), or nitrogen-vacancy-hydrogen (NVH) forms (Edmonds et al. 2012). Since CVD diamond is grown at low temperatures (well below its Debye temperature), is produced in relatively short time scales (∼ days to months, not years), and has a comparatively low nitrogen concentration, there is little opportunity to form the more complicated nitrogen-related defects common to natural diamonds, such as the A- (nitrogen pairs), B- (N₄V⁰) aggregates, or N3 (N₃V⁰) centers, unless subjected to post-growth annealing at high temperatures.

A source of atomic hydrogen is a critical component in the CVD growth of diamond materials. In the commercial production of CVD diamond, two general sources of atomic hydrogen are employed: plasmas and hot wires or surfaces (at over 1800 °C). Each approach has technical advantages and disadvantages, as well as economic and intellectual property concerns. Almost all growth of single crystal and high purity material is done with plasma assisted CVD, generally driven by microwave sources (primarily at 2.45 GHz or 915 MHz), although some manufacturers use lower frequencies and even DC driven plasmas. Figure 3 shows a schematic of a microwave CVD reactor, while Figure 24 displays a picture of a single seed diamond growth in a typical commercial reactor operating at 2.45 GHz. Figure 25 displays a multiple seed diamond growth run in a different CVD reactor. Hot wire (tungsten, tantalum, etc.) methods of dissociating hydrogen are used mostly for growth covering larger areas (up to 1 m²), thin film, and polycrystalline coatings. CVD diamond growth has also been demonstrated in combustion flames and molecular beams (Hansen et al. 1988; Lee et al. 1994), but neither of these is currently used in commercial production.

Many companies are involved in the commercial production of CVD diamond products. Due to the competitive nature of the business, it is difficult to determine the exact size and nature of the production factories and few detailed images are available. Figure 26 displays two examples of parts of CVD diamond materials growth facilities.

There are a wide range of applications for diamond materials in technology due to its many extreme properties (thermal, mechanical, optical, etc.). Figure 27 displays images of many of the applications of CVD diamond materials. A unique advantage of CVD diamond is the ability to control and monitor the growth process, to “engineer” the material to the application. This can be done by controlling the purity, grain size, thermal diffusivity, optical transparency, toughness, where and what dopants are introduced, surface termination, isotopic purity, etc. The diverse array of applications of CVD diamond span the fields of mechanical, optical, electronic, medical, quantum, electrochemical, communications, and high-power electronics, to name a few. These have been reviewed in many places such as (Prelas et al. 1997; Butler and Windischmann 1998; Wort and Balmer 2008; Balmer et al. 2009; Auciello and Sumant 2010; Nemanich et al. 2014; Awschalom et al. 2018; Yang et al. 2019; Achard et al. 2020).

The largest widely publicized initial commercial application for blocks of single crystal diamond grown by CVD methods is for gems. Figure 28 displays a 9.04 ct round brilliant cut CVD diamond, while Figure 29 shows a unique all-diamond ring fabricated from a single block of CVD diamond. As of early 2022, a 16.41 ct (G color, VVS₂ clarity) princess-cut specimen is the largest reported gem CVD diamond (Wang et al. 2022). Major players in CVD diamond gem production can be found in the United States, United Kingdom, India, Singapore and China (Linde et al. 2021).

As the quality and purity of laboratory-grown diamonds have improved over the years, visual inspection of polished diamonds—in particular if colorless or near-colorless—is rarely sufficient for their identification as to origin. Instead, conclusive identification is largely based on the presence, absence, relative concentrations and distributions of point defects in the diamond lattice, detected by advanced microscopic and spectroscopic means. The formation of these defects are a result of the growth environment and associated processes as well as any subsequent residence at elevated temperatures and/or treatment conditions, consequently providing an insight into the diamond in question’s history. It is important to note that most point defects are not unique to either natural or laboratory-grown diamonds. Laboratory-grown diamonds may also be treated to further expand the types of possible defects. Thus care needs to be taken when combining several pieces of evidence to reach a conclusion on a diamond’s natural or laboratory-grown origin.

Certain defects, termed color centers, absorb electromagnetic radiation in the visible range, influencing the color of the diamond material if present in sufficiently high concentrations. Both natural and laboratory-grown diamonds exist in the full rainbow spectrum of colors. Collins (1982, 1992, 1993a, 2001), Ashfold et al. (2020), and Green et al. (2022, this volume) review some of the main color-producing defects in diamonds. Defects in natural and laboratory-grown diamonds may be as-grown or produced through treatments such as HPHT or low pressure high temperature (LPHT) annealing, irradiation, or combinations thereof. In the gem trade natural diamonds are most commonly found in hues ranging from colorless to light yellow, and are typically characterized by letter color grades ranging from D-to-Z, respectively, based on the diamond color grading system invented by the GIA (King et al. 2008). Brown and gray diamonds that are more colorless than a K color reference master stone are also assigned letter grades. For color saturations equivalent to the grades K-to-Z, brown-hued diamonds are given a letter grade as well as a color word description (K–M = “Faint,” N–R = “Very Light,” and S–Z = “Light”), whereas gray diamonds are only given word descriptions. Diamonds with stronger yellow color saturation than the letter grade Z, or other color hues, are known as “fancy color” diamonds and are described using the fancy color grade terminology as explained by King et al. (1994). Certain gemological laboratories offer laboratory-grown diamond reports, characterizing the materials using conventional diamond grading boundaries and terminology.

In the following sections, the identification tools used by gemologist and scientists for diamond analysis will be summarized. Subsequently, the signature characteristics of both HPHT- and CVD-grown diamonds will be reviewed, highlighting the features that can be used for their identification.

The identification of natural and laboratory-grown diamonds relies on a range of different observations, with gemological laboratories developing confidential testing protocols to protect the gem and jewelry buying public from deceptive or unintentional misrepresentation of laboratory-grown diamonds as natural. The tools used for identification can be separated into observational and spectroscopic tools, with many of the former being the building blocks of standard gemological studies. Foremost amongst them is a microscope allowing visual inspection of samples under magnification (10–64× for most gemological applications) with reflected, bright field and dark field illumination capabilities (Renfro 2015). Coupled with crossed-polarized filter plates, birefringence patterns related to crystallographic strain in the diamond can be revealed, which may be indicative of certain growth or post-growth processes. These patterns are also sometimes referred to as anomalous double refraction patterns (Kane 1980). Howell (2012) presents a comprehensive review on strain birefringence in natural diamonds.

Defects in diamond can interact with incident light through a variety of processes which can allow their detection (Collins 1982, 1992, 1993a; Ashfold et al. 2020; Green et al. 2022, this volume). Zaitsev (2001) and the references therein catalog several hundred features associated with defects in diamond materials, with numerous defect structures identified. Table 1 provides a brief summary of some of the key features discussed in this chapter. Many defects absorb light at characteristic wavelengths in the ultraviolet (UV) to visible and infrared (IR) range, and can be detected using absorption spectroscopy. Once appropriately normalized, the integrated intensity of the absorption spectrum for a specific defect is proportional to its concentration, with defect-dependent detection limits that may be as low as a few parts per billion carbon atoms (Collins and Williams 1971; Davies 1981; Kiflawi et al. 1994; Boyd et al. 1994, 1995; Lawson et al. 1998; Howell et al. 2019). Typically, fast Fourier transform infrared (FTIR) absorption spectra are collected at room temperature. The sensitivity of UV-visible absorption is greatly improved by cooling the samples to liquid nitrogen temperature (77 K).

Defect absorptions from nitrogen- and boron-related defects in the IR range are the basis for a diamond classification system (Robertson et al. 1934), where Type I diamonds contain detectable nitrogen, in either the aggregated (Type Ia) or isolated (Type Ib) forms. The aggregated nitrogen in Type Ia diamonds may be in the A- or B-center forms (nearest neighbor nitrogen pairs or N₄V⁰, respectively) with the type refined into Type IaA, IaB or IaAB (Davies 1976, 1981; Jones et al. 1992; Boyd et al. 1994, 1995). Isolated substitutional nitrogen is also referred to as the C-center (Type Ib) (Dyer et al. 1965). If nitrogen is below the detection limit, it is a Type II diamond, further divided into those that do (Type IIb) or do not (Type IIa) contain detectable boron. The IR features associated with these defects are listed in Table 1. For more on the diamond classification system and its use in diamond gemology, refer to Collins (1993a), Breeding and Shigley (2009), Ashfold et al. (2020), and Green et al. (2022, this volume).

The absorption of light may also stimulate luminescence, the emission of light from defects in the material (Collins 1993a; Eaton-Magaña and Breeding 2016; Green et al. 2022, this volume). Conventionally, diamond luminescence is evaluated by observing the sample using a shortwave (254 nm) and longwave (365 nm) UV combination gem lamp, preferably in a dark room (Luo and Breeding 2013). The luminescence color (wavelength and energy) is related to the defect species that are effectively stimulated, and their distribution through the sample may also be observed. Luminescence detected while the sample is under illumination is known as fluorescence or photoluminescence, whereas phosphorescence is the emission of light after the excitation source is turned off. The fluorescence and phosphorescence response of diamonds depend upon the defects present in the material, and as such vary for the different synthesis chemistries, potential treatments, diamond types, and colors. Further detail can be recognized using an instrument such as the DiamondView, where deep-UV illumination with energies greater than the diamond bandgap (i.e., wavelengths <225 nm, energies >5.5 eV) is coupled with video imaging under magnification. Nearly all diamonds will fluoresce under these conditions, and the resulting patterns will be sharp and surface-specific (Welbourn et al. 1996). These patterns are primarily associated with changes in the defect concentrations as the diamonds grow, whether naturally or in a laboratory, and as such are excellent features for their distinction. Importantly, although treatments may modify the fluorescent defect concentrations, and thus potentially the colors and distributions observed, the fundamental growth pattern will remain. Several viewing angles should be used when attempting to observe fluorescence patterns.

Additionally, gemological laboratories often use photoluminescence (PL) spectroscopy at 77 K to confirm the presence of specific luminescent defects in diamond, allowing sensitivities which may be of the order of parts per trillion. This is particularly useful for defect analysis of colorless diamonds, which have low concentrations of color centers. Unlike absorption spectroscopy, the relationship between the intensity of luminescent features and their concentration is complex, with the presence of certain defects (e.g., A-centers) potentially quenching the luminescence of others (e.g., Vasil’ev et al. 2004). A color center’s excitation efficiency also depends upon the illuminating laser wavelength, so spectra are typically collected using a suite of different lasers with wavelengths such as 325, 488, 514, 532, 633, 830 nm. More rarely, cathodoluminescence imaging and spectroscopy, where incident electrons are used to excite luminescence, can also be used for defect analysis.

External morphology. Gem quality HPHT diamonds are grown using the temperature gradient method, where the morphology is sensitive to growth conditions such as the temperature, pressure, and metal solvent/catalyst, dictating which crystallographic faces will dominate (Strong and Chrenko 1971; Kanda et al. 1980, 1989; Burns et al. 1999). HPHT diamond growth originates at a diamond seed crystal, simultaneously growing upwards and outwards along several crystallographic directions, dominated by growth along the cube 〈100〉 and octahedral 〈111〉 directions. Consequently HPHT diamonds typically exhibit well-developed octahedral {111} and cube {100} growth faces, along with minor dodecahedral {110} and trapezohedral {113} faces, as shown in Figures 6 (Strong and Chrenko 1971; Kanda et al. 1980, 1989; Sunagawa 1984, 1995; Shigley et al. 1986, 1987, 1992, 1993a, b; Burns et al. 1999). {115} and {117} faces have also been reported (Strong and Chrenko 1971; Rooney 1992; Shigley et al. 1992). The distinctive cuboctahedral morphology can be used to distinguish HPHT crystals from natural diamonds, which generally have octahedral, resorbed, or irregular shapes (Sunagawa 1995; Harris et al. 2022, this volume). However, occasionally HPHT diamonds can also be irregularly shaped.

Dendritic, vein-like, spiral, and lamellar or striated surface patterns can be observed on the external faces (Tolansky and Sunagawa 1959; Kanda et al. 1980; Koivula and Fryer 1984; Shigley et al. 1986, 1987, 1992, 1997; Sunagawa 1995). In particular, the dendritic surface patterns are considered to be indicative of HPHT diamonds. Unlike many natural rough diamonds these crystals do not show significant mechanical abrasion or chemical etching. Though rarely seen, triangular depressions (commonly known as “trigons”) or triangular pyramids may be present on octahedral {111} faces and are thus not exclusive to natural diamonds (Kanda et al. 1980; Koivula and Fryer 1984; Shigley et al. 1987, 1993a, 1997; Rooney 1992). The surface patterns on HPHT diamonds have been attributed to phase transformations and the microstructure of the metal solvent/catalyst during the final quenching stages of synthesis (Kanda et al. 1980; Burns et al. 1999). The bottom growth surface of the crystal may bear the imprint of the seed.

Growth sectors. A growth sector is defined as the region of the sample that was grown with a common crystallographic growth plane (Burns et al. 1990), as illustrated in Figure 30. The incorporation efficiency for impurities differs for the growth sectors, leading to sector-dependent distributions throughout the crystal (Strong and Chrenko 1971; Burns et al. 1990, 1999; Collins et al. 1990; Satoh et al. 1990; Lawson et al. 1996). For instance, for most as-grown HPHT diamonds isolated nitrogen defect concentrations in the sectors can be arranged from highest to lowest as {111} > {100} > {113} > {110} (Burns et al. 1990). The behavior may be reversed for {100} and {111} sectors when grown at low temperatures (Satoh et al. 1990). Meanwhile for boron, another common contaminant or dopant of HPHT diamonds, the highest concentrations are found in the {111} sectors, followed by the {110} sectors (Burns et al. 1990). The distribution of nickel- and cobalt-related defects have also been found to be sector-dependent (Collins et al. 1990; Lawson et al. 1996). Importantly, as this distribution is the result of the intrinsic growth structure, it can still be perceived if the sample has been cut and polished to remove the distinctive external morphology. Furthermore, although defect species may be modified through treatment, the growth pattern will remain (Shigley et al. 1993a, 2004a; Moses et al. 1993). Consequently, the positive identification of HPHT origin is often based on techniques that can reveal the distinguishing growth pattern.

HPHT diamonds with sufficiently high concentrations of color-producing defects can show distinct color zoning (Figs. 16 and 31). For cut samples, the appearance will depend not only on the defect concentrations of the growth sectors, but also on their distribution relative to the faceting arrangements. The zoning may be more obvious by viewing samples under magnification and diffuse lighting while immersed in methylene iodide, water, or colorless alcohol to minimize reflections. There are many examples of brown-to-yellow-to-orange color zoning due to single substitutional nitrogen, and blue due to boron, documented in the literature (Crowningshield 1971; Strong and Chrenko 1971; Koivula and Fryer 1984; Shigley et al. 1986, 1987, 1992, 1993a, 2002, 2004a,b; Burns et al. 1990; Fritsch and Shigley 1993; Rooney et al. 1993; Howell et al. 2019). Mixed Type Ib + IIb HPHT diamonds may show both yellow and blue color zoning, and may have a green face-up appearance if faceted (Shigley et al. 1992, 2004a; Fritsch and Shigley 1993; Rooney et al. 1993; Hainschwang and Katrusha 2003). Rarely, the concentration of nickel-related defects in HPHT diamonds can be sufficiently high to produce green color, preferentially incorporating into the {111} growth sectors (Collins and Spear 1982; Collins et al. 1990; Johnson and Myagkaya 2017; Eaton-Magaña 2019). Red or pink hued HPHT diamonds can be created by irradiation followed by annealing of Type Ib or Ib + IaA HPHT material, producing high concentrations of nitrogen-vacancy (NV) centers responsible for the color (Moses et al. 1993; Shigley et al. 2004a). Color zoning is not exclusive to HPHT diamonds. For instance, ~8% of natural yellow diamonds have a patchy “scotch and water” color distribution (King et al. 2005), while pink diamonds often show pink lamellae (Hofer 1985; King et al. 2002; Gaillou et al. 2010; Eaton-Magaña et al. 2018a), and subtle blue parallel bands may be observed in blue diamonds (King et al. 1998). Treatments that include irradiation can also produce regional color concentrations in both natural and laboratory-grown samples (Fritsch and Shigley 1989; Wang et al. 2005a). However, sharp geometric patterns—in particular squares, octagonal “stop signs,” and the “Maltese Cross” (Figs. 30c,d)—are rarely observed in natural diamonds and should be treated as suspicious.

In addition to color zoning, “color graining” may be observed at the growth sector boundaries in the interior of the diamond or on its surface (Fig. 32). As with color zoning, graining may reveal a stop-sign, Maltese Cross, funnel, or hourglass patterns (Shigley et al. 1986, 1987, 1992, 1993a, 2004a; Fritsch and Shigley 1993). Grain lines may also be found in the {100} sectors in planes aligned parallel to the cube crystal faces (Shigley et al. 1987, 1992). In contrast, grain lines in natural diamonds are usually parallel to octagonal planes (Kane 1980). For brown-to-yellow-to-orange HPHT diamonds, this distinctive graining is attributed to variations in the index of refraction from the changing nitrogen content (Burns et al. 1990; Frank et al. 1990). For a review on graining in natural diamonds see Kane (1980).

By definition, near-colorless, and colorless HPHT diamonds cannot show color zoning. However, the underlying growth sectors are still present, and can be detected using techniques that are more sensitive to minute defect concentrations, such as deep UV fluorescence imaging. The application of fluorescence methods to reveal growth zoning in both colored and colorless HPHT diamonds will be detailed below in Luminescence behavior.

Inclusions. Like natural diamonds, HPHT diamonds are susceptible to trapping materials from their environment during growth. Traces of the metal solvent/catalyst can be encapsulated in the crystal, often having elongated, triangular tabular, or irregular shapes, as shown in Figures 33a,b (Crowningshield 1971; Strong and Chrenko 1971; Koivula and Fryer 1984; Shigley et al. 1986, 1987, 1992, 1993a,b, 1997, 2002, 2004a,b; Burns et al. 1990, 1999; Fritsch and Shigley 1993; Moses et al. 1993; Lang et al. 1995; Sumiya and Satoh 1996; Sumiya et al. 2002; D’Haenens-Johansson et al. 2014, 2015a). Using transmitted light, they are black and opaque, while reflected light reveals a gray and metallic appearance. The metal solvent/catalyst composition, growth rate, temperature, and pressure can affect the number, size, and shapes of the inclusions. The addition of nitrogen getters to exclude nitrogen from the growing diamond by forming stable nitrides can further increase the uptake of inclusions (Sumiya and Satoh 1996; Burns et al. 1999, 2009; Sumiya et al. 2002). Due to their ferrous origin, an inclusion-containing HPHT diamond may be attracted to a strong magnet (Koivula and Fryer 1984; Rossman and Kirschvink 1984; Shigley et al. 1986, 1987, 1993a,b, 1997, 2004a,b; D’Haenens-Johansson et al. 2014, 2015a; Eaton-Magaña et al. 2017). For example, D’Haenens-Johansson et al. (2014, 2015a) found that faceted HPHT diamonds with clarities of VS₂ or poorer were weakly magnetic. Rare metallic inclusions have been observed in Type IIa, IaB, and IIb natural diamonds, but differ in appearance, with natural inclusions found adjacent to graphitic decompression cracks unlike those in the laboratory-grown samples (Sharp 1966; Rossman and Kirschvink 1984; Jacob et al. 2004; Kaminsky and Wirth 2011; Mikhail et al. 2014; Smith et al. 2016, 2018a,b).

Dilutely dispersed whitish pinpoint inclusions, as shown in Figure 33c, are frequently seen in fancy colored HPHT diamonds (Crowningshield 1971; Koivula and Fryer 1984; Shigley et al. 1986, 1987, 1993a,b, 1997, 2002, 2004a). Their sparse, even distribution is distinct from that of pinpoints in natural diamonds, which are typically concentrated in wispy clouds or associated with growth zones. Pinpoints in HPHT diamonds may consist of metallic and silicate phases (Lang et al. 1995). In diamonds grown using nitrogen getters, pinpoints have been attributed to the formation of stable carbides associated with the nitrogen getter element (Sumiya and Satoh 1996; Burns et al. 1999). The addition of copper to the growth capsule can suppress carbide formation.

Figure 33d presents another unusual inclusion seen in HPHT diamonds, described as a “horsetail” (Crowningshield 1971; Koivula and Fryer 1984). They form near the crystal surfaces and are typically lost during faceting.

Strain birefringence patterns. Examination of untreated near-colorless, colorless, and blue HPHT diamonds (which are always Type II, see the Absorption spectroscopy and color section) between crossed-polarized filters has been shown to be a simple method to alert the viewer of their laboratory-grown nature. Their low concentration of impurities, coupled with the even pressure that they are subjected to during growth, result in high crystalline perfection and few extended defects such as dislocations (Sumiya et al. 1997; Burns et al. 2009; Sumiya and Tamasaku 2012; Tran Thi et al. 2017). Consequently, they have low internal strain levels and only show extremely weak (black, gray or blue interference colors)—or more typically no—strain birefringence patterns, except adjacent to inclusions and cracks (Fig. 34) (Crowningshield 1971; Lang et al. 1991; Fritsch and Shigley 1993; Rooney et al. 1993; Shigley et al. 1993b; D’Haenens-Johansson 2014, 2015a). Natural diamonds commonly show mottled, banded, radial, or tatami (cross-hatched) birefringence patterns originating from the presence of defects, inclusions, and deformation events from their growth and mantle residency history (Tolansky 1966; Lang 1967; Kane 1980; Sumiya et al. 1997; Howell 2012). Natural Type II diamonds are characterized by high levels of strain and dislocations, forming distinctive tatami patterns. Conversely, natural Type I diamonds occasionally do not show clear strain patterns, so the absence of a pattern in an unknown diamond is not conclusive proof that it is HPHT-grown, and should thus be analyzed using another complementary method for confirmation (e.g., FTIR for diamond type) (Koivula and Fryer 1984). Radial, cross-hatched, and banded strain birefringence patterns have been reported for Type I HPHT diamonds due to variations in the lattice parameters from the inhomogeneous distribution of nitrogen defects, as well as surrounding both pinpoint and larger inclusions (Koivula and Fryer 1984; Shigley et al. 1986, 1987, 1992, 1993a, 2004b; Frank et al. 1990; Lang et al. 1991; Fritsch and Shigley 1993; Sumiya et al. 1997; Palyanov et al. 2010). Many note that the patterns are nevertheless weaker compared to those for natural diamonds.

Luminescence behavior.Table 2 summarizes common fluorescence and phosphorescence colors and intensities for as-grown and treated HPHT diamonds analyzed using shortwave and longwave UV (Shigley et al. 2004b). Representative data for individual samples can be found in Crowningshield (1971), Koivula and Fryer (1984), Shigley et al. (1986, 1987, 1992, 1993a,b, 1997, 2002, 2004a), Moses et al. (1993), Rooney et al. (1993), and D’Haenens-Johansson et al. (2014, 2015a), while Eaton-Magaña et al. (2017) present statistical analysis of several thousand HPHT diamonds submitted to GIA. Most HPHT diamonds show a stronger fluorescence response to the shortwave rather than the longwave UV light produced by a gem lamp, a useful quality as it is opposite to that typical to natural diamonds (e.g., Moses et al. 1997; King et al. 1998, 2002, 2005). It is important to note that there are exceptions to this behavior for both HPHT and natural diamonds. The sector-dependent impurity incorporation in HPHT diamonds can lead to zonation in the fluorescence patterns, often taking distinctive square or cross shapes which may be perceived in fancy colored specimens (Fig. 35a). Cuboctahedral patterns are extremely rare for natural diamonds.

Colorless to near-colorless HPHT diamond are typically inert to longwave UV light, with Eaton-Magaña et al. (2017) reporting only ~2% of colorless HPHT diamonds showing fluorescence, which was orange or yellow in color. Yet among natural diamonds, 35% show longwave UV fluorescence, and of those 99% fluoresce blue due to the presence of N3 (N₃V⁰) centers (Moses et al. 1997). The strong, persistent phosphorescence observed for colorless to near-colorless HPHT Type IIa and IIb diamonds is also particularly useful for their detection due to the rarity of phosphorescence in similarly colored natural diamonds (Fig. 35b) (Crowningshield 1971; Koivula and Fryer 1984; Rooney et al. 1993; Shigley et al. 1993b, 1997; D’Haenens-Johansson et al. 2014, 2015a). The phosphorescence, associated with boron (Watanabe et al. 1997), indicates the presence of this element even in nominally Type IIa HPHT diamonds. This behavior is the basis for certain diamond screening equipment used by gemologists. However, this useful property can be removed by irradiation treatments (Robinson 2018).

Inspection under deep-UV (wavelength <225 nm) can reveal further detail in the growth patterns, stimulating sufficient contrast even in near-colorless to colorless Type IIa or weak Type IIb HPHT diamonds such as those shown in Figure 36 (Welbourn et al. 1996; Shigley et al. 1997, 2002, 2004b; D’Haenens-Johansson et al. 2014, 2015a; Eaton-Magaña et al. 2017). The majority of natural diamonds show octahedral, or occasionally cubic, growth patterns which may also reveal a complex sequence of dissolution and growth events (Welbourn et al. 1996; Harris et al. 2022, this volume). In contrast, cuboctahedral patterns such as those seen for HPHT diamonds are unusual. Growth zones are rarely detected for natural Type II diamonds, whose fluorescence patterns are instead dominated by polygonized dislocation networks (Fig. 36d) and can thus be clearly separated from HPHT diamonds (Sumida and Lang 1981; Martineau et al. 2004). The fluorescence colors depend upon the defect compositions of individual samples. When considering colorless and near-colorless natural and HPHT-grown Type II diamonds it is common for naturals to show clear blue “Band A” fluorescence associated with dislocations or N3 centers (Dean 1965; Ruan et al. 1992; Takeuchi et al. 2001; Martineau et al. 2004), whereas HPHT diamonds show blue-green or green-blue fluorescence colors (D’Haenens-Johansson et al. 2014, 2015a). Furthermore, deep-UV light stimulates strong, long-lived green-blue phosphorescence centered about 500 nm and may also show yellow emission at 575 nm (Watanabe et al. 1997; Eaton-Magaña et al. 2008; Eaton-Magaña and Lu 2011; D’Haenens-Johansson et al. 2014, 2015a). Under deep-UV, the overall phosphorescence of natural Type IIb diamonds may appear blue, gray, violet, or red due to emission bands centered about 500 nm and/or 660 nm, decaying more rapidly than that of laboratory-grown diamonds (Eaton-Magaña et al. 2008; Eaton-Magaña and Lu 2011).

Largely superseded by the comparatively user-friendly DiamondView deep-UV imaging system, cathodoluminescence has also been shown to be a powerful technique for detailed analysis of HPHT diamond growth patterns and their luminescent defects (Woods and Lang 1975; Shigley et al. 1987, 1992, 1993a,b; Collins et al. 1990; Ponahlo 1992; Fritsch and Shigley 1993; Rooney et al. 1993; Burns et al. 2009).

Absorption spectroscopy and color. HPHT diamonds belonging to several classification types can be produced (Table 2). The different impurity uptakes of growth sectors mean that oftentimes a single crystal will contain minor sectors belonging to different diamond types. Unless steps are taken to minimize nitrogen incorporation standard HPHT synthesis results in predominantly Type Ib material, similar to that mass-produced for diamond grit (Dyer et al. 1965; Chrenko et al. 1971; Crowningshield 1971; Strong and Chrenko 1971; Koivula and Fryer 1984; Shigley et al. 1986, 1987, 1992, 1993a, 2002, 2004a,b; Burns et al. 1990; Fritsch and Shigley 1993; Rooney et al. 1993). They may have yellow, to greenish or orangy or brownish yellow, to orange-yellow color. Type Ib diamonds contain single substitutional nitrogen dispersed through the crystal lattice. This color center produces absorption both in the IR and visible regions. IR absorption spectra show a peak at 1344 cm^–1 and a broad band at 1130 cm^–1 (Dyer et al. 1965; Chrenko et al. 1971; Collins 1980; Collins and Woods 1982; Kiflawi et al. 1994; Lawson et al. 1998), while UV-visible spectra show a rise in absorption from ~560 nm toward the intrinsic diamond absorption edge at 225 nm, with a broad peak at 270 nm, responsible for the yellow-to-orange-to-brown color (e.g., Fig. 12) (Dyer et al. 1965; Chrenko et al. 1971; Collins 1982, 1993a, 2001). Nickel-related defects introduced by the metal solvent/catalyst may produce additional absorption features at 494, 658, 793.5 (with vibronic structure at ~700 nm), and a doublet about 884 nm (broad band at ~685 nm), imparting a green hue (Collins and Spear 1982; Shigley et al. 1987, 1993a, 2004a; Collins et al. 1990; Lawson and Kanda 1993a,b; Yelisseyev and Nadolinny 1995; Yelisseyev et al. 1996, 2002; Yelisseyev and Kanda 2007; Johnson and Myagkaya 2017; Eaton-Magaña 2019). These peaks are not exclusive to HPHT diamonds, but may indicate a laboratory-grown diamond that needs further inspection (e.g., Chalain 2003; Lang et al. 2004; Wang and Moses 2007; Wang et al. 2007a; Breeding et al. 2018). Figure 37a illustrates a representative UV-visible absorption spectrum for a Type Ib yellow HPHT diamond. In nature, Type Ib diamonds are very rare, accounting for ~0.1% of diamonds, and often contain natural needle-like inclusions to help in their identification (Dyer et al. 1965; Shigley et al. 1986; Breeding and Shigley 2009). Most yellow-hued natural diamonds are Type Ia and are instead colored by the “Cape series” of peaks that include the N3 (N₃V⁰) zero phonon line at 415 nm, vibronic structure to shorter wavelengths, as well as the N2 transition at the same defect at 478 nm (Davies et al. 1978; Collins 1982, 2001; King et al. 2005; Green et al. 2022, this volume).

Isolated nitrogen atoms may aggregate to A-centers (nitrogen pairs) if synthesis times and temperatures are elevated, resulting in Type IaA material, and may also progress further to form B-centers (N₄V⁰) (Kanda et al. 1990; Vins et al. 1991; Kanda and Yamaoka 1993; Shigley et al. 1993a, 2002; Babich et al. 2000, 2004; Yelisseyev et al. 2002). HPHT or LPHT annealing of as-grown Type Ib diamond is an alternative route for nitrogen aggregation (Chrenko et al. 1977; Evans et al. 1981; Evans and Qi 1982; Shigley et al. 1993a; Kiflawi et al. 1994, 1998; Kim et al. 2011; Kazuchits et al. 2016; Chen et al. 2018). For both the as-grown and treated cases, a portion of the isolated nitrogen centers will remain (e.g., they are mixed Type Ib + IaA) as the nitrogen aggregates cycle through formation and dissociation (Brozel et al. 1978). A- and B-centers show characteristic IR spectra with absorption features at 1282 and 1175 cm^–1, respectively (Davies 1981; Boyd et al. 1994, 1995). They do not produce diamond color as they do not absorb light in the visible range. However, the isolated nitrogen present will contribute, resulting in similarly hued material as the Type Ib HPHT diamonds (Fig. 37a) (Shigley et al. 1993a, 2002; Yelisseyev et al. 2002; Kim et al. 2011; Kazuchits et al. 2016; Chen et al. 2018). Unlike natural diamonds, there are no Type Ia colorless or near-colorless HPHT diamonds. This is significant as ~98% of natural diamonds are Type Ia (Dyer et al. 1965), making diamond typing a useful tool for screening out colorless to near-colorless HPHT diamonds. Nitrogen-containing HPHT diamonds that have been annealed or grown at temperatures sufficiently high to induce nitrogen aggregation can also form the N3 and N₃VH⁰ (3107 cm^–1) centers (Chrenko et al. 1977; Evans and Qi, 1982; Kiflawi et al. 1996; Goss et al. 2014). These centers are common in natural Type Ia diamonds, and occasionally observed in Type IIa diamonds.

Type Ib and Ib + IaA HPHT diamonds are candidates for irradiation and annealing to form pink/red to orangy or brownish pink/red to purplish pink or purple material, as show in Figure 15 (Moses et al. 1993; Shigley et al. 2004a,b). This treatment sequence combines vacancies produced by the irradiation with the resident isolated nitrogen centers to form nitrogen-vacancy (NV) centers. NV centers in the neutral and negative charge states absorb in the visible, with zero phonon lines at 575 and 637 nm, respectively (Collins 1982, 2001). Due to the high isolated nitrogen content in Type Ib and Ib + IaA, which acts as an electron donor, a majority of the NV centers will be negatively charged, resulting in absorption from its associated vibronic structure between 500 and 600 nm, producing the pink-to-red hued color component (Fig. 37b). This, combined with the yellow-to-brown component of the isolated nitrogen absorption produces the characteristic color range of these treated HPHT diamonds. The treatment may also introduce features including GR1 (neutral vacancy, V⁰, 741 nm), H3 (N₂V⁰, 503.2 nm), and 595 nm peaks as tabulated by Moses et al. (1993) and Shigley et al. (2004a). In the infrared range, absorption from the dinitrogen 〈001〉 split-interstitial H1a (1450 cm⁻¹) and a peak at 1502 cm⁻¹, both produced through irradiation and annealing treatment, may be detected (Clark et al. 1956; Collins 1980; Woods and Collins 1982; Woods 1984; Collins et al. 1988; Liggins 2010; Liggins et al. 2010). Increased control during growth and treatment can also be used produce weak Type Ib (nominally Type IIa) pink HPHT diamonds (Eaton-Magaña et al. 2017). The vast majority of similarly hued natural diamonds are colored by unidentified broad absorption bands at 390 and 550 nm, an underlying brown absorption continuum attributed to vacancy clusters, as well as possibly N3 (Collins 1982, 2001; King et al. 2002; Fisher et al. 2009; Gaillou et al. 2010; Byrne et al. 2012, 2014a,b; Howell et al. 2015; Eaton-Magaña et al. 2018a, 2020; Green et al. 2022, this volume), making the detection of NV centers suspicious. Untreated natural Type IIa pink diamonds colored by NV centers, known as “Golconda” diamonds, only account for ~0.6% of natural pink diamonds (Fritsch 1998; Eaton-Magaña et al. 2018a). However, note that NV centers can also be responsible for pink color in treated natural and CVD diamonds (Collins 1982, 2001; Wang et al. 2005a, 2010; Eaton-Magaña and Shigley 2016).

Blue or grayish blue HPHT diamonds containing sufficient uncompensated boron (i.e., an excess of boron compared to isolated nitrogen) to be classified as Type IIb show absorption features at ~2458, 2803, and 2930 cm⁻¹, with the latter feature having a tail which extends through the infrared into the red region of the visible spectrum (Collins and Williams 1971; Strong and Chrenko 1971; Koivula and Fryer 1984; Collins 1993b; Rooney et al. 1993; Shigley et al. 2004a,b; Howell et al. 2019; Green et al. 2022, this volume). Boron readily incorporates into the growing diamond lattice, and may be added intentionally to the growth capsule. Boron contamination may originate from the carbon source material or the metal solvent/catalyst (Sumiya and Satoh 1996). Colorless and near-colorless HPHT diamonds can be Type IIa or IIa + IIb, where the uncompensated boron concentration is insufficient to produce appreciable color (Strong and Chrenko 1971; Koivula and Fryer 1984; Rooney et al. 1993; Shigley et al. 1993b, 1997, 2004b; Sumiya and Satoh 1996; D’Haenens-Johansson et al. 2014, 2015a). Pure Type IIa diamonds only show intrinsic diamond absorption in the infrared region, and are transparent through the visible to UV range down to the diamond absorption edge at 225 nm. Figure 37 shows a representative UV-visible absorption spectrum for both blue (d) and colorless (e) HPHT diamond, demonstrating an absence of sharp features. Their absorption spectra are often indistinguishable to those of natural Type IIa and IIb diamonds, highly valuable groups that account for <1.3% and <0.02% of natural gem diamonds, respectively (King et al. 1998; Smith et al. 2016, 2018a; Eaton-Magaña et al. 2018a).

Samples have also been grown containing both Type IIb and Ib sectors (Shigley et al. 1992, 2004a,b; Fritsch and Shigley 1993; Rooney et al. 1993; Hainschwang and Katrusha 2003). When faceted they may have a yellowish green to green to bluish green appearance due to the absorption contributions from boron and isolated nitrogen (Fig. 37). The authors are not aware of any reports of mixed Type Ib + IIb natural diamonds, in agreement with Shigley et al. (1992).

Type IIa and weak Ib HPHT diamonds may be irradiated to produce vacancies, introducing GR1 (V⁰) absorption with a zero phonon line at 741 nm and vibronic structure in the red, and may also include sharp irradiation-related features such as ND1 (V^–, 393.5 nm), GR2–GR8 peaks (412–430 nm), and the 595 nm peak (Fig. 37c) (Davies 1977; Shigley et al. 2004a,b). The treated diamond will have a yellowish green to green to bluish green to blue color, depending on the strength of nitrogen-related absorption present in the as-grown material and the ratio of GR1 to ND1 defects. The GR1 center is also the primary defect responsible for green color in both natural and treated diamonds, complicating their distinction (Collins 1982, 2001; Shigley and Fritsch 1990; Breeding et al. 2018; Green et al. 2022, this volume). Natural diamonds colored by GR1 may be Type IIa or Ia. For a review on green color in natural diamonds, including spectroscopic and gemological features such as irradiation spots or coatings, see Breeding et al. (2018).

Photoluminescence spectroscopy. The increased detection sensitivity provided by PL is particularly useful for probing defects in colorless to near-colorless Type IIa or IIb diamonds. Nevertheless, PL spectra for such HPHT diamonds typically show very few, and oftentimes weak, emission peaks (D’Haenens-Johansson et al. 2014, 2015a). The most prominent features are nickel-related, such as the 884 nm doublet and a 4-peak multiplet at about 484 nm (Collins and Spear 1982, 1983; Davies et al. 1989; Collins et al. 1990; Nazaré et al. 1991, 1995; Lawson and Kanda 1993a,b; Collins 2000; Yelisseyev and Kanda 2007; D’Haenens-Johansson et al. 2014, 2015a), likely due to nickel contamination from the metal solvent/catalyst. Occasionally doublet emission at 736.7/736.9 nm from the negatively charged silicon-vacancy center (SiV^–) is detected (Clark et al. 1995, D’Haenens-Johansson et al. 2014, 2015a), a defect more commonly associated with CVD diamonds (Wang et al. 2003, 2005b, 2007b, 2012; Martineau et al. 2004). These centers are seldom observed for natural diamonds, and are thus useful indicators of HPHT synthesis (Chalain 2003; Noble et al. 1998; Wang and Moses 2007; Wang et al. 2007a; Breeding and Wang 2008).

Absorption spectroscopy, coupled with gemological observations and fluorescence behavior provide sufficient information to support the identification of fancy colored HPHT diamonds. Many of the features that would be observed using PL can be detected in absorption. Consequently, there are not many detailed reports presenting their PL spectra for this purpose (Shigley et al. 2002, 2004a; Kennedy and Johnson 2016; Eaton-Magaña et al. 2017). Most notable are peaks associated with nickel-related defects (484 nm; S2/NE2, 489.2 nm; S3/NE1, 496.7 nm; NE3, 523.3 nm; 658 nm; 694 nm; NE8, 793.5 nm; 884 nm), nitrogen-related defects (H3/N₂V⁰, 503.2 nm; NV⁰, 575 nm; NV^–, 637 nm; H2/N₂V^–, 986 nm), or the neutral vacancy defect GR1 (741 nm) if the sample had undergone irradiation treatment (Yelisseyev et al. 1996, 2002, 2003, 2006; Kanda and Watanabe 1999; Nadolinny et al. 1999; Yelisseyev and Kanda 2007). Peak details are summarized in Table 1.

External morphology. As-grown uncut CVD diamonds can be readily identified due to their unique appearance, as shown in Figure 22. CVD diamond morphology depends upon the orientation of the underlying seed and growth conditions (Silva et al. 2006, 2008, 2009; Brinza et al. 2008; Tallaire et al. 2013). Gem-quality material is usually synthesized using (100)-oriented substrates, consequently growing as cubic (001)-oriented thick films with (100) and (010) edges. Minor dodecahedral {110} faces may form along the edges, as well as octahedral {111} and trapezohedral {113} faces on the crystal corners. The {111} faces are undesirable as they are susceptible to twinning, resulting in stress buildup and crack development. The {110} faces have also been linked to higher internal stress levels and sample breakage (Tallaire et al. 2013). Uncut CVD diamonds may also show a dark polycrystalline diamond rim along the edges (Mokuno et al. 2005; Nad et al. 2015, 2016). The top surface topography of a CVD crystal is affected by many factors including the substrate orientation, misalignment angle, and crystalline quality, gas composition, and temperature, and may show macro-bunching steps that flow across the surface and/or hillocks (Janssen et al. 1992; de Theije et al. 2000; Takeuchi et al. 2000; Wang et al. 2005b; Achard et al. 2007; Martineau et al. 2009).

Visual features. CVD diamonds generally obtain high clarities. They may contain small, black, irregularly shaped amorphous carbon or disordered graphitic inclusions or pinpoints (Wang et al. 2003, 2005b, 2007b, 2012; Martineau et al. 2004; Crisci et al. 2011). Sometimes these inclusions are cloud-like or lie in planes oriented parallel to the substrate. In samples that have been HPHT treated for decolorization, inclusions may be surrounded by radial fractures stemming from high levels of localized strain (Wang et al. 2012). Inclusions in CVD diamonds are unfortunately not diagnostic since they have a similar appearance to those in some natural diamonds.

Color concentration is generally even in fancy colored pink or brown CVD diamonds, though fluctuations in growth parameters or interruptions may lead to colored bands aligned parallel to the substrate plane (Wang et al. 2003, 2005b, 2010; Martineau et al. 2004). Graining associated with threading dislocations may be apparent (Wang et al. 2007b, 2010).

Strain birefringence patterns. Threading dislocations aligned approximately perpendicular to the growing surface are known to nucleate at or near the interface between the substrate and the CVD diamonds, often forming bundles (Fujita et al. 2006; Gaukroger et al. 2008; Martineau et al. 2009; Pinto and Jones 2009). They may lead to an anisotropic strain birefringence, most easily observed parallel to the growth direction in unfaceted samples (Martineau et al. 2004). Irregular, banded, and cross-hatched (tatami-like) birefringence patterns have been seen for CVD diamonds (Fig. 38), with interference colors ranging from subdued grays and blues to vivid reds and greens (Wang et al. 2003, 2005b, 2007b, 2010, 2012). Localized strain fields may be found surrounding inclusions (Wang et al. 2003, 2005b, 2010, 2012; Pinto and Jones 2009; Crisci et al. 2011). Unlike many HPHT diamonds, strain birefringence patterns in CVD diamonds are generally difficult to distinguish from those in natural samples.

Luminescence behavior. Most colorless and near-colorless CVD diamonds are inert to longwave UV light. Analysis of CVD diamonds submitted to GIA between 2003 and 2016 suggests only approximately 7% of samples show very faint to faint fluorescence (Eaton-Magaña and Shigley 2016). However, it was noted that the incidence of longwave UV fluorescence decreased over the years as CVD products evolved, with all samples tested since 2013 being inert. When the color of such fluorescence was noted, as-grown CVD diamonds emitted orange, orange-yellow, or yellow light originating from nitrogen-vacancy centers (Martineau et al. 2004; Wang et al. 2005b, 2007b). The brown hue common in CVD diamonds grown with intentional or unintentional nitrogen doping can be removed by high temperature annealing (usually using an HPHT press, but LPHT annealing has also been reported), resulting in a more valuable gem product (Wang et al. 2003, 2012; Charles et al. 2004; Martineau et al. 2004; Mäki et al. 2007; Meng et al. 2008; Liang et al. 2009; Eaton-Magaña and Shigley 2016; Eaton-Magaña et al. 2021). This treatment will reduce NV content, while introducing H3 (N₂V⁰) centers, producing green fluorescence (Wang et al. 2012). These orange and green fluorescence colors can alert viewers of their laboratory-grown origin as 99% of natural similarly colored fluorescing diamonds emit blue light due to N3 (N₃V⁰) (Moses et al. 1997). Furthermore, CVD diamonds show a stronger fluorescence response to shortwave UV, in contrast to most naturals.

The second most common color of CVD diamonds are pink-hued (Eaton-Magaña and Shigley 2016; Eaton-Magaña et al. 2021), with the vast majority colored by high concentrations of NV centers produced through treatment (see Absorption spectroscopy and color section below). The dominance of NV centers in pink CVD diamonds gives rise to moderate to strong red or orange fluorescence to both longwave and shortwave UV light (Wang et al. 2010; Eaton-Magaña and Shigley 2016). Extremely rare Golconda pink natural diamonds (~0.6% of natural pink diamonds) as well as most treated pink natural-grown and HPHT diamonds (Table 2) will also fluoresce in similar hues due to their elevated NV concentrations (Eaton-Magaña and Shigley 2016; Eaton-Magaña et al. 2018a). Meanwhile, 58% and 9% of natural pink diamonds colored by the 550 nm band show blue (N3) or yellow fluorescence, respectively, with the remainder being inert (Eaton-Magaña et al. 2018a).

Inspection of CVD diamonds under magnification and deep-UV excitation (<225 nm) can reveal characteristic growth patterns and help with conclusive identification. CVD synthesis of gem-quality diamond usually proceeds through step-flow growth on (001)-oriented substrates, with riser and terrace segments oriented along [001] and [101] directions. As they present differently oriented planes of growth, the distinct impurity uptake of terraces and risers can result in the formation of a fluorescence pattern of tightly spaced lines known as “striations,” illustrated in Figures 39 and 40 (Martineau et al. 2004, 2009). Additionally, deliberate or accidental growth events such as variations in deposition conditions or interruptions can dramatically modify the impurity content, resulting in an abrupt change in fluorescence color and/or brightness (Figs. 39b and 40a,d). In a cross-sectional view this will appear as a line lying parallel to the growth plane. Evidence of these growth events, in particular in larger CVD diamonds, suggests that they are being grown in stages as a series of layers (Mokuno et al. 2005; Asmussen et al. 2008; Dieck et al. 2015; Nad et al. 2015; Law and Wang 2016). Figure 22 displays an as-grown CVD diamond crystal grown in two stages. Blue fluorescing threading dislocations, typically nucleating at or close to the boundary between the substrate and CVD diamond layer, may propagate through the CVD material aligned approximately parallel to the bulk growth direction (Martineau et al. 2004). Step-flow growth may cause the dislocations to deviate, following a zigzag path (Martineau et al. 2009). Depending on the viewing direction, dislocations may appear as threads or bundles with a clear linear habit, or may be patchy (Martineau et al. 2004, 2009; Wang et al. 2005b; Dieck et al. 2015). The anisotropy of the patterns emphasizes the importance of inspecting diamonds from multiple viewing angles.

Deep-UV fluorescence images for faceted CVD diamonds are presented in Figure 40. Variations in growth and treatment histories mean that CVD diamond fluorescence can span the full rainbow spectrum of colors (Eaton-Magaña and Shigley 2016). Importantly, treatments cannot remove the intrinsic growth patterns of diamonds, but will affect the relative concentrations of defects and thus potentially fluorescence colors. Here we review the most commonly observed fluorescence reactions. High purity CVD diamonds with low nitrogen concentrations are characterized by low fluorescent defect content, typically only showing patchy or thread-like blue fluorescence from dislocation bundles, yet can be separated from natural Type IIa polygonized dislocation networks by careful examination (Martineau et al. 2004; Wang et al. 2005b). Such CVD diamonds are rarely grown for gem purposes due to cost. Instead, deep-UV fluorescence images of as-grown colorless, near-colorless or brown CVD diamonds usually show orange, red or pink bulk fluorescence due to NV center emission (Wang et al. 2003, 2005b, 2007b; Martineau et al. 2004; Wassell et al. 2018; McGuinness et al. 2020). They may also weakly phosphoresce orange or blue (Wang et al. 2007b; McGuinness et al. 2020). HPHT annealing for decolorization introduces H3 (N₂V⁰), N3 (N₃V⁰), and a series of unidentified emission peaks and bands between 430 and 500 nm (Wang et al. 2012; Wassell et al. 2018; McGuinness et al. 2020). Consequently, the overall fluorescence excited by deep-UV changes from the as-grown orange, red, or pink response colors to green or green-blue. Additionally, the HPHT annealed diamonds show strong green-blue phosphorescence that may be associated with N–B donor–acceptor pairs (Watanabe et al. 1997; Wang et al. 2007b, 2012; Wassell et al. 2018). Multi-treatment of CVD diamonds to produce pink body color yield high NV concentrations, resulting in intense yellowish orange, orange, reddish orange, or red fluorescence (Wang et al. 2007b, 2010).

Absorption spectroscopy and color. The vast majority of D–Z color CVD diamonds can be classified as Type IIa, although trace amounts of isolated nitrogen may be detected for poorer colors (N_S⁰ peak at 1344 cm⁻¹). The rarity of this diamond type in nature, and its frequency for both HPHT- and CVD-grown diamonds emphasize that such diamonds should be treated as suspicious and undergo careful testing. FTIR spectra may aid in determining whether a CVD diamond is as-grown or treated. Hydrogen-related peaks at 8753, 7354, 6856, 6425 (also reported at 6428 cm^–1), 5564, and 3323 cm⁻¹ (N–C–H, also reported at 3324 cm^–1) are exclusive to CVD diamonds, but can be reduced or removed by LPHT or HPHT decolorizing treatments (Fuchs et al. 1995a,b; Glover 2003; Charles et al. 2004; Martineau et al. 2004; Cruddace 2007; Meng et al. 2008; Liang et al. 2009; Liggins 2010). The 3123 cm⁻¹ peak, identified as the neutral nitrogen-vacancy-hydrogen center (NVH⁰), is also common in CVD diamonds (Fuchs et al. 1995a,b; Cruddace 2007; Khan et al. 2013). It may be useful for identification as it has not been observed for Type IIa natural diamonds, yet may be detected in some Type Ia diamonds with intense N₃VH⁰ center absorption (3107 cm⁻¹) (Lai et al. 2020). NVH⁰ also anneals out during HPHT treatment, but may survive LPHT treatment (Martineau et al. 2004; Cruddace 2007; Meng et al. 2008; Liang et al. 2009). The N₃VH⁰ center, a common defect in natural diamonds, can be introduced by annealing at temperatures >1700 °C (Charles et al. 2004; Martineau et al. 2004; Liggins 2010; Wang et al. 2012). For more detail regarding FTIR absorption features present in as-grown and HPHT treated D–Z color CVD diamonds see the work by Martineau et al. (2004) and Wang et al. (2003, 2005b, 2007b, 2012).

UV-visible-NIR absorption spectra of CVD diamonds may have a combination of broad bands and sharper peaks contributing to their color (Fig. 41). At room temperature, colorless and near-colorless CVD diamonds are often featureless, possibly showing an absorption ramp rising from ~1200 nm to the diamond absorption edge at 225 nm. The underlying “profile” is proportional to λ^–3 (Khan et al. 2013). The defect responsible for this gradual increase in absorption is still unclear, though it may be caused by vacancy clusters or defects with internal surfaces with sp²-like bonding (Barnes et al. 2006; Mäki et al. 2007; Jones 2009). Thus the absorption spectra may appear similar to those of rare natural Type IIa diamonds, yet differ from those of the much more common Type Ia diamonds with Cape features. Poorer color as-grown CVD diamonds tend to have yellow, brown, or pinkish undertones, with the hue determined by the relative intensities of broad absorption features at ~270 (N_S⁰), 360, 520 nm (related to NVH⁰, and sometimes referred to as the 515 nm band), and the underlying profile (Martineau et al. 2004; Khan et al. 2009, 2010, 2013). Khan et al. (2009, 2010, 2013) demonstrated that these three bands are thermo- and photochromic, weakening (becoming more colorless) after heating at 525–550 °C due to charge transfer involving N_S^0/+, NVH^0/–, and other currently unidentified defects. This effect is reversible, with UV light stimulating their increase. Annealing at temperatures greater than 1400 °C causes permanent changes to the defect structures, reducing the intensities of the featureless absorption profile, as well as the 360 and 520 nm bands, and decreasing the brown color saturation. Differences in their thermal stabilities mean that annealing at temperatures greater than 1800 °C—at which high pressures are typically used to minimize sample graphitization—results in more efficient color removal (Khan et al. 2013).

Absorption spectroscopy with cooling to liquid nitrogen temperatures may reveal the presence of additional defects, including H3 (N₂V⁰, 503.2 nm), NV⁰ (575 nm), NV^– (637 nm), SiV^– (736.7/736.9 nm), and an unknown CVD-specific center absorbing at 596 nm, depending on the growth and treatment history (Wang et al. 2003, 2005b, 2007b, 2012; Martineau et al. 2004). SiV^– can be detected using the more sensitive photoluminescence technique in a few natural (<1%) and HPHT diamonds (Breeding and Wang 2008; D’Haenens-Johansson et al. 2014, 2015a), but has not been reported using absorption methods, rendering it a good indicator of CVD diamond material. A peak at 946 nm has been identified as the neutral charge state of the silicon-vacancy (SiV⁰), but is seldom detected (D’Haenens-Johansson et al. 2010, 2011; Breeze et al. 2020). Silicon is often unintentionally introduced into the growing CVD material by plasma etching of silicon-containing reactor components such as silica windows (Robins et al. 1989; Barjon et al. 2005). Unlike many other indicator features in CVD diamond, silicon-vacancies have excellent thermal stability and only start to anneal out at 2200 °C (Clark and Dickerson 1991). Thus SiV^– may be detected in CVD diamonds of many colors.

CVD diamonds that have been selected for irradiation and annealing treatments to produce pink color from NV centers are very weak Type Ib (sometimes referred to as “nominally Type IIa” by the gemological community), showing a sharp peak at 1344 cm⁻¹ and broad band at 1130 cm⁻¹ (both from N_S⁰) in the IR. Their absorption features are similar to those observed for pink treated HPHT diamonds. The UV-visible-NIR absorption spectra (Fig. 41) are dominated by NV centers, but also show other nitrogen-related features such as the 270 nm broad band from N_S⁰ and the 503.2 nm peak from H3 (N₂V⁰). Additionally, they include features associated with irradiation such as ND1 (V^–_, 393.5 nm), GR1 (V⁰, 741 nm), or irradiation and annealing such as the “595 nm” (594.3 nm) defect (Wang et al. 2010; Zaitsev et al. 2018; Song et al. 2021). FTIR spectra further indicate treatment, containing the dinitrogen 〈001〉 split-interstitial H1a (1450 cm⁻¹) and the 1502 cm⁻¹ peak (Wang et al. 2010; Song et al. 2021). As “NV-pink” CVD samples are nitrogen-containing, the CVD-specific 3123 cm⁻¹ (NVH⁰) peak may be detected (Wang et al. 2010; Zaitsev et al. 2018). However, it is worth noting that this feature will be absent or weakened in samples that underwent HPHT treatment for brown color removal prior to irradiation and annealing (Wang et al. 2010; Song et al. 2021). Diamonds colored pink by NV centers are likely to be treated natural or laboratory-grown diamonds and thus should be thoroughly tested (Moses et al. 1993; Shigley et al. 2004a,b; Wang et al. 2005a,b, Eaton-Magaña et al. 2018a). Eaton-Magaña and Shigley (2016) present a succinct table comparing characteristics for such samples.

Unlike natural and HPHT diamonds, blue gem-quality CVD diamonds are rarely Type IIb crystals colored by boron. Controlled boron doping can be achieved by the addition of boron containing chemicals such as diborane (B₂H₆) to the growth gases, but synthesis is challenged by poor crystal morphology, low growth rates, and soot formation at high doping levels (Achard et al. 2012). Martineau et al. (2004) reported growing experimental boron-doped CVD diamonds with an inhomogeneous layered appearance. These samples were Fancy Light grayish blue to Fancy Deep blue in color, showed absorption spectra similar to boron-containing natural diamonds (e.g., King et al. 1998), and did not show nitrogen, hydrogen or silicon-related features to help identify their laboratory-grown origin. Boron-doped CVD overgrowth layers on natural diamonds have been observed at GIA, where 80–200 µm CVD diamond were deposited onto the stones to impart a blue color when viewed face-up. In absorption, these hybrid diamonds share properties of both the underlying natural diamond and the CVD layer, allowing identification, especially when combined with fluorescence imaging (Fritsch and Phelps 1993; Moe et al. 2017; Ardon and McElhenny 2019).

Instead of boron-doping, the most popular route to create blue colored CVD diamonds is by post-growth irradiation treatment of Type IIa samples (Ardon and Wang 2014; Moe et al. 2015; Eaton-Magaña 2018; Song et al. 2020). This introduces the GR1 (V⁰) defect with a peak at 741 nm and vibronic structure absorbing in the red, as well as irradiation features such as TR12 (469.9 nm) and the 595 nm center. Irradiated CVD diamonds have a green modifier, with the color described as greenish blue or green blue, hues which account for less than 9% of blue natural diamonds (Eaton-Magaña et al. 2018b).

Unusual gray to blue color due to strong absorption from the SiV⁰ center and its associated photoconductivity spectrum has also been reported for highly silicon-doped CVD diamonds (Peretti et al. 2014; D’Haenens-Johansson et al. 2015b; Eaton-Magaña and Shigley 2016; Breeze et al. 2020; Eaton-Magaña et al. 2021). SiV⁰ and SiV^– are photo- and thermochromic, where the blue color can be temporarily deepened by UV illumination. Exposure to strong white light returns such diamonds to their base color, which may be gray, brownish pink or less intense blue.

Photoluminescence spectroscopy. Photoluminescence spectra collected at liquid nitrogen temperature are particularly useful for CVD diamond identification (e.g., Fig. 42), though multiple laser excitations are necessary to detect defects spanning the UV to NIR region. As-grown CVD spectra typically show emission from NV⁰ (575 nm), NV^– (637 nm), often SiV^– (736.7/736.9 nm), and occasionally SiV⁰ (946 nm), H3 (N₂V⁰, 503.2 nm), and a peak at 389 nm (Iakoubovskii et al. 2000; Wang et al. 2003, 2005b, 2007b; Charles et al. 2004; Martineau et al. 2004; Eaton-Magaña and Shigley 2016). The latter peak, thought to be nitrogen-related, has been reported in some HPHT diamonds and in irradiated nitrogen-containing diamonds (Collins and Woods 1987; Collins and Lawson 1989; Robins et al. 1989; Zaitsev 2000; Martineau et al. 2004). CVD-specific PL features include peaks at 468 nm (also reported at 467 nm) and 533 nm (or 534 nm), and a doublet at 596/597 nm. The center responsible for the 468 nm peak (also referred to as TR12’) may involve nitrogen as the peak’s intensity increases with nitrogen concentration, but has also been attributed to interstitial- or vacancy-cluster-related defects (Iakoubovskii and Adriaenssens 2000a,b; Iakoubovskii et al. 2000; Rzepka et al. 2001; Zaitsev 2001; Iakoubovskii and Stesmans 2002; Zaitsev et al. 2021). The 533 nm peak may originate from a nitrogen-vacancy type complex (Ruan et al. 1991; Zaitsev 2001). The defect emitting at 596/597 nm is unassigned, but is thought to differ from that associated with the 596 nm absorption peak in brown CVD diamonds (Martineau et al. 2004).

HPHT annealing destroys the defects responsible for the 389, 468, 533, and 596/597 nm features, while also reducing the intensity of NV centers (Charles et al. 2004; Martineau et al. 2004; Wang et al. 2012; Zaitsev et al. 2021). Yet these conditions result in the formation of more complex nitrogen-related defects such as N3 (N₃V⁰, 415 nm), H3 (N₂V⁰, 503.2 nm), and its negatively charged counterpart, H2 (986 nm). Additionally, the treatment introduces unidentified peaks at 850 and 875.5 nm, and a series of lines in the 451–459 and 520–580 nm regions.

The PL spectra for CVD diamonds that have been treated pink by irradiation and annealing for nitrogen-vacancy formation are unsurprisingly dominated by emission from both NV⁰ and NV^– centers, but also include peaks at 389, 495.9 (H4, N₄V⁰), 503.5 (3H, interstitial-related), 796.9, 806.4, and 949.0 nm (Wang et al. 2010; Eaton-Magaña and Shigley 2016; Song et al. 2021). Strong absorption from the “595 nm” (594.3 nm) and GR1 (V⁰, 741 nm) centers, which overlap with the vibronic bands associated with NV⁰ and NV^– luminescence, are detected as negative peaks. Further peaks will be present as outlined above, depending on whether the material was as-grown or underwent HPHT treatment prior to irradiation and annealing.

The development and global expansion of diamond synthesis using HPHT and CVD technologies has led to rapid improvements in material quality and sizes. These dynamic and competitive fields continue to be actively researched by academic and industrial scientists alike, who characterize diamond and its crystal defects to exploit many of its unrivalled properties and find new avenues of use. The nascent laboratory-grown gem diamond sector is showing signs of maturity, benefiting from decades of synthesis research for industrial and technological applications. Despite the similarities between natural and laboratory-grown diamonds, the fundamental differences in their formation provide evidence of their growth origin, enabling their conclusive identification.

UFSDJ would like to thank the diamond research and gemology team members at GIA, whose comprehensive data collection and examination of both natural and laboratory-grown diamonds are the basis of the identification section. Also, she would like to acknowledge the work by Alex Balter (GIA) who conducted a lot of the referenced statistical analysis of diamonds submitted to our laboratories. JEB thanks the CVD diamond research community for many years of open, supportive, and collaborative discussions which rapidly advanced our understanding prior to the current commercialization efforts. ANK is deeply grateful to all the specialists who contributed to the fascinating research process of growing new and ultra large diamond crystals. Special recognition go to Mr. Roman Isakov, the Chief Technology Officer of New Diamond Technology Ltd. (NDT), for his exceptional engineering talent in the diamond production field, and to Dr. Alexander Koliadin, the former CTO of NDT, whose innovative ideas made it possible to start the growth project. Thank you also to all the teams at NDT and Alkor-D Ltd. who daily provide the technological support necessary to create world record-breaking laboratory-grown diamonds. The authors are also thankful to Dr. Sally Eaton-Magaña and Dr. Christopher M. Breeding for providing helpful comments on an early draft of the manuscript. Dr. Evan M. Smith is thanked for help with Figure 1. We are also grateful for reviews by Prof. Michael Ashfold, Dr. Boris N. Feigelson, and Dr. Daniel J. Twitchen.