Brown-to-gray and near-colorless single-crystal type IIa synthetic diamonds grown using a chemical vapor deposition (CVD) technique by Apollo Diamond Inc. have gemological properties that are distinct from those of both natural diamonds and HPHT-grown synthetic gem diamonds.  The tabular crystals typically range up to 1 ct or more and a few millimeters thick, and consist of an overgrowth on a natural or synthetic diamond substrate. Faceted CVD synthetic diamond usually cannot be separated from natural diamond with standard gemological techniques; although when portions of the substrate are still present, an experienced gemologist may be able to recognize differences in luminescence or color between the overgrowth and substrate.  In all cases, however, the CVD synthetic diamonds examined to date could be identified in a gemological laboratory by the combination of a strong orangy red fluorescence seen with the De Beers DiamondView deep ultraviolet imaging system, a characteristic anomalous birefringence (strain) pattern, and distinctive features in their infrared absorption spectra (e.g., at 3123 cm^-1) and photoluminescence spectra (strong 575 and 637 nm emissions, a doublet at 596 and 597 nm, and a line at 737 nm).

Composite of photos by Elizabeth Schrader