Fall 2011

Featured Lab Note: Gem-Quality CVD Synthetic Diamonds from Gemesis

In recent years, limited quantities of gem-quality synthetic diamonds produced by chemical vapor deposition (CVD) have reached the market. Most of these have come from Apollo Diamond Inc. (Boston, Massa­chusetts). We recently examined similar products introduced by Florida-based Gemesis Corp., better known for its HPHT-grown synthetic diamonds (J. E. Shigley et al., “Gemesis laboratory-created diamonds,” Winter 2002 G&G, pp. 301–309). In November 2010, the company announced plans to market CVD-grown synthetics.

GIA examined 16 CVD synthetic diamonds that were faceted as round brilliants with the exception of one rectangular cut. They ranged from 0.24 to 0.90 ct, with an average weight of 0.46 ct. They were colorless (3 samples), near-colorless (11), and lightly colored (2). For the most part, clarity grades fell between IF and VVS, due to internal graining and a few pinpoint inclusions. Only four of the 16 samples had VS clarity.

Infrared absorption spectroscopy revealed that all of the CVD synthetic diamonds were type IIa. Unlike typical as-grown CVD products, no defect-related absorptions were recorded in either the mid- or near-infrared regions. Photoluminescence analysis of all samples showed moderately strong emission from the H3 optical center (zero-phonon line at 503.2 nm), moderately strong emissions from NV centers (575.0 and 637.0 nm), and moderate to strong emissions from the [Si-V]^– center (doublet at 736.6 and 736.9 nm). In four samples, the [Si-V]^– concentrations were relatively high and could even be detected in the UV-Vis absorption spectra. A notable feature of these synthetic diamonds was their weak to moderate green fluorescence to short-wave UV radiation. Most were inert to long-wave UV radiation, with only five samples showing very weak green fluorescence. In the Diamond­View, they exhibited strong green fluorescence and noticeable blue phosphorescence; characteristic growth striations also were easily seen.

Gemological and spectroscopic observations strongly suggested that these CVD synthetic diamonds were annealed after their growth, presumably to improve their color and transparency. This study confirms that the quality of CVD synthetic diamonds continues to improve. Nevertheless, gemological and spectroscopic features can clearly separate Gemesis CVD synthetics from natural diamonds.

Wuyi Wang and Thomas M. Moses