Annealing of gem-quality diamonds at very high pressures (above 5 GPa) and temperatures (above ∼1800°C) can produce significant changes in their color. Treatment under these high-pressure-high-temperature (HPHT) conditions affects certain optically active defects and their absorptions in the visible spectrum. In the jewelry industry, laboratory-treated diamonds are valued much less than those of natural color. Polished diamonds are carefully examined at gemological laboratories to determine the “origin of color” as part of an overall assessment of their quality. Currently, the recognition of HPHT-treated diamonds involves the determination of various visual properties (such as color and features seen under magnification), as well as characterization by several spectroscopic techniques. HPHT-treated diamonds were introduced into the jewelry trade in the late 1990s, and despite progress in their recognition, their identification remains a challenge. While some detection methodologies have been established, the large number of diamonds requiring testing with sophisticated analytical instrumentation poses a logistical problem for gemological laboratories.