Diamond is the most illustrious of all minerals. Made of pure carbon atoms packed into a dense isometric structure, diamond is treasured as a gemstone for its brilliant (adamantine) luster and supreme hardness, and is in demand for many industrial applications because of these same properties. On Earth, most natural diamonds form deep (>160 km) under continental cratons, where high static pressures (>45 kilobars) stabilize diamond relative to graphite, which is the low-pressure polymorph of pure carbon. Such diamonds are only fortuitously brought to the surface, carried as xenoliths or xenocrysts in explosive volcanic pipes known as kimberlites. To geoscientists, diamonds provide invaluable records of the extreme conditions and otherwise inaccessible environments in which they formed. Diamonds can also probe exotic extraterrestrial environments. Nanometer-sized diamonds also occur as a rare component of the most primitive carbonaceous chondrite meteorites, rocks which preserve the original components of the Solar System. These nanodiamonds are pre-solar in origin, most likely formed by low-pressure condensation in other stellar environments and then incorporated into the solar nebula (Bernatowicz et al. 2006). Nanometer-sized diamonds are also found in the Canyon Diablo iron meteorite, which created the 1.2 km wide Meteor Crater (Arizona, USA). These diamonds are restricted to specimens that were collected near the rim of the crater and show evidence of reheating, which argues that they formed by shock transformation from preexisting carbon phases when the meteor impacted Earth (Lipschutz and Anders 1960). Few other meteorites, or meteorite types, have been reported to contain diamonds … with one renowned exception: the curious case of diamonds in ureilites.

The ureilites constitute a major group of nonchondritic meteorites. They are coarse-grained ultramafic rocks consisting mostly of olivine and pyroxene and are interpreted to represent the residual mantle of an asteroid that was partially melted and differentiated ≤1 My after formation of the Solar System. Their silicate minerals record igneous temperatures up to 1,250–1,280 °C. One of the characteristic properties of ureilites is that they have very high carbon contents (up to 8 wt%; average ~3.5 wt%), comparable to those of the most carbon-rich carbonaceous chondrites. The carbon occurs predominantly as well-crystallized graphite, typically associated with Fe–Ni metal and sulfide, located along silicate grain boundaries (Fig. 1). The diamonds occur as a volumetrically minor component embedded in the graphite. The origin of diamonds in ureilites has been debated for more than 60 years. Urey (1956) suggested that they formed under static high pressures deep in a large, at least Moon-sized, parent body, analogous to terrestrial diamonds. However, based on the three ureilites that were known at that time, Lipschutz (1964) argued that the properties of the diamonds (nanometer-sized and showing preferred orientation) indicated formation by shock transformation from graphite in an extraterrestrial impact event, possibly during breakup of the parent body. Since these first studies, the number of ureilites has increased dramatically (>600 to date), expanding the range of their petrologic properties and allowing application of a shock scale based on their silicate minerals. It is now clear that although the first known ureilites were highly shocked, many others show lower shock levels. Diamonds in ureilites exhibit a variety of features, and more than 25 research papers have been devoted to investigating their origin. The vast majority of studies have supported the impact shock hypothesis, although a third alternative—chemical deposition in the solar nebula—has also been proposed (see summary in Nestola et al. 2020). The “large parent body hypothesis” remained out of favor for >50 years, based not only on properties of the diamonds themselves but also on the prevailing view that the asteroidal parent bodies of meteorites were too small (less than a few hundred kilometers in diameter) to reach diamond-forming pressures in their interiors.

Perversely, the large parent body hypothesis for the origin of diamonds in ureilites was revived by Miyahara et al. (2015) and Nabiei et al. (2018), who studied diamonds in ureilitic clast MS-170 from the Almahata Sitta (AhS) polymict ureilite and claimed that the clast was a unique sample from a low-shock level in which primary features were preserved. Miyahara et al. (2015) found that diamonds are particularly abundant in the carbon areas of this ureilite and often occur as clusters of isolated, tens of micrometer-sized grains, all having similar crystal-lographic orientations. They interpreted these clusters as remnants of former single crystals up to 100 micrometers in size. They then argued that such large diamonds could not have grown in the short duration (microseconds) during which peak pressures were maintained in an impact event and so they must have formed under static high pressures deep in the ureilite parent body. Nabiei et al. (2018) concluded that this body must have been at least Mars-sized, and was, therefore, one of the “lost” planetary embryos whose existence in the early Solar System has been predicted by recent planetary formation models. A new twist was added to the story by Desch et al. (2019), who accepted the conclusion that ureilite diamonds formed under high static pressures (Nabiei et al. 2018) but proposed that, instead of being endogenous, they formed in the interior of Mars. They were then ejected from Mars in fragments liberated by the Borealis impact, one fragment then colliding with the ureilite parent body and becoming intermixed with ureilitic material.

These new arguments for the formation of ureilite diamonds at high static pressures were challenged by Nestola et al. (2020) who investigated diamonds in several ureilites, including MS-170. Admittedly, a low shock level in MS-170 might facilitate preservation of primary diamonds, evidence of which had been destroyed in more highly shocked samples. However, Nestola et al. (2020) discovered actual single crystal diamonds ≥100 micrometers in size in the highly shocked ureilite NWA 7983 (Fig. 2). The large diamonds coexist with the nanodiamonds, which are the expected product of shock, suggesting the possibility that the large diamonds were also formed by the shock process rather than simply having survived it. Nestola et al. (2020) pointed out that shock formation of diamond in ureilites would have been catalyzed by the Fe–Ni–C metal that is ubiquitously associated with carbon in ureilites: this effect is commonly exploited in industrial diamond synthesis. Nestola et al. (2020) calculated that peak shock pressures could have lasted 4–5 seconds during the major impact that disrupted the ureilite parent body, and they showed that this is sufficient for metal-catalyzed growth of 100 micrometer-sized diamonds.

Nestola et al. (2020) also pointed out that ureilite diamonds fail to show the octahedral morphology of diamond crystals grown over long periods of time under high static pressures. In ureilites of very low shock level (Fig. 1), the graphite occurs as millimeter-sized euhedral (blade-shaped or tabular) crystals showing prominent (0001) cleavage. Notably, diamonds have not been found in these very low-shock samples. In more highly shocked ureilites, the carbon areas retain these blade-shaped external morphologies (Figs. 3 and 4) and the diamonds are embedded in graphite that is now polycrystalline. In fact, Nestola et al. (2020) found that both the nanodiamond aggregates and the large diamonds in these blade-shaped carbon areas in NWA 7983 show a prominent striped texture, parallel to the long dimension of the laths, which likely represents the trace of (0001) in the original graphite (Fig. 3). These observations strongly suggest that the diamonds are pseudomorphing original graphite crystals and formed in a rapid process that did not allow time for the external morphology of graphite to be replaced by that of diamond. Goodrich et al. (2020) pointed out that this observation pertains to MS-170 as well, i.e., the carbon areas in which the diamonds are embedded have elongated, blade-shaped morphologies (Fig. 4). If the aggregates of similarly oriented diamond grains in these areas are remnants of larger diamonds formed by slow growth under static high-pressure conditions (Miyahara et al. 2015), their external morphologies should be those of diamond rather than of graphite. Finally, Nestola et al. (2020) noted that the formation of diamonds in MS-170 by impact shock is consistent with the shock level of this sample (S3), which is not, in fact, especially low for a ureilite.

The work of Nestola et al. (2020) and Goodrich et al. (2020) casts reasonable doubt on purported evidence for diamond formation in ureilites under static high-pressure conditions and provides continued support for diamond formation during impact shock events, as concluded by many previous researchers. Nevertheless, additional investigations, utilizing state-of-the-art techniques, could further test this hypothesis. For example, aggregates of similarly oriented small diamonds in MS-170 may not be remnants of single large crystals (Miyahara et al. 2015). Similar aggregates are typically formed during shock compression of oriented graphite, leading to a well-defined crystallographic relationship between the diamonds and the original graphite. Electron backscattered diffraction techniques could be used to efficiently determine whether aggregates of diamonds having a similar orientation over large areas show this relationship. The external morphologies of the carbon areas could also be used to constrain the orientation of the original graphite. A comprehensive survey of the properties of diamonds as a function of shock level in ureilites has yet to be done.

The origin of diamonds in ureilites will continue to be debated, along with other mysteries about these meteorites. Not least of which is where the carbon itself originated.