DIAMOND PROVENANCE DETERMINED BY MULTIVARIATE ANALYSIS OF LIBS SPECTRA: NEW INSIGHTS TO DIAMOND CHEMISTRY

Increased consumer confidence in the country of origin of diamonds is possible only with accurate provenance determination using information naturally associated with each stone. Multivariate analysis of LIBS (Laser-Induced Breakdown Spectroscopy) spectra has been demonstrated as a rapid, accurate, and minimally destructive method to determine the mine of origin for diamonds. This study reports aspects of diamond chemistry recorded in LIBS spectra on which provenance determination is based. Thirty rough, inclusion-free, diamonds from each of 10 diamond mines worldwide and 30 cut and polished synthetic diamonds produced by each of two different processes were used. All were cleaned by industry standards and with isopropyl alcohol immediately prior to analysis. LIBS spectra were acquired in an ultra-pure Ar atmosphere in four 4X4 grids placed randomly on diamond surfaces, sampling different crystallographic directions. Spectra were analyzed by the multivariate Quantagenetics© method (Patents 9,063,085 and 8,699,022) based on Bayesian statistics using the Euclidian distance between an unknown spectrum and the mean spectrum of each group. The percent of correctly classified spectra ranges from 95.8% to 100.0% for individual localities, and 98.5% overall. Success rates for Bayesian-based statistical systems increase as the sample base grows.

Multivariate analysis reveals the chemical traits that vary in the groups of diamonds. Synthetic diamond spectra record peaks of C, Mg, Fe, Si, Ti, N, Ca, Na, H, O and molecules C-N and C-C. These stones represent the interior portion of the diamond, suggesting that these elements exist within the diamond structure as defect centers rather than as surface contamination. Defect centers related to Fe, Ni, Co, and Mn have been reported in synthetic diamond (Iakoubovskii and Adriaenssens, 2002a, b). The natural diamonds in this study contain Co, Cu, Ni, and Al in addition to the elements in synthetic diamonds. Molecular emission from C-C and C-N dimers formed in the plasma are also important in determining provenance; Serrano et al. (2015) demonstrated that the electronic structure of atoms in a solid affects the relative intensities of molecular emissions. Synthetic and natural inclusion-free diamonds contain sufficient chemical variation for provenance determination.