CARBON ISOTOPE ANALYSIS OF CVD SYNTHETIC GEM DIAMONDS

Quality of single-crystal CVD diamond has improved significantly. Many are introduced to the jewelry market, generating tremendous concern. The ability to confidently separate synthetics from natural ones is critical for consumer confidence. Current identification methods focus on optical properties, including types and concentrations of impurities or combinations of impurities and their distribution in the diamond lattice. This approach has proven very successful for separating CVD synthetics from HPHT synthetics and natural diamonds. However, continuous improvement of CVD growth technology and, in particular, post-growth treatments introduces new identification challenges. In an effort to proactively confront these challenges, we present a preliminary evaluation of carbon isotope analysis as a tool for comparison of CVD synthetic diamonds with natural ones.

19 CVD diamonds from three makers (Gemesis=7; Scio=11; Element Six=1) were analyzed by SIMS. Multiple spots (5-10) were analyzed for each crystal. Unlike natural diamonds, which typically show heterogeneous carbon isotope distributions, the CVD synthetics exhibited very homogeneous isotope compositions. Standard deviation among multiple analyses on each sample was less than 0.6‰, close to the instrument analytical uncertainty. d¹³C of CVD diamonds from Scio were in the range -35 to -45‰, whereas samples from Gemesis were -55 to -75‰. The single sample from Element Six was -63.7‰, consistent with the Gemesis range. These results strongly suggest that CVD synthetic diamonds are much lighter in carbon isotope composition than natural diamonds, which occur in the range d¹³C = -10 to 0‰ (peridotitic) or extend to values as low as -30‰ (eclogitic). Effectively, no overlap is observed in carbon isotope chemistry between natural and CVD diamonds. In combination with the homogenous distribution of carbon isotopes in CVD diamonds, this separation in d¹³C values could be a very useful feature for separating natural diamonds from CVD synthetics. Very large differences were revealed between CVD diamonds from different manufacturers, as well as among CVD diamonds from the same manufacturer. Variations in CH₄ gas chemistry, different carbon sources, or growth conditions (P, T, growth rate) may have contributed to fractionation of carbon isotopes.