PRECISELY IDENTIFYING THE MINES FROM WHICH GEMSTONES WERE EXTRACTED: A CASE STUDY OF COLOMBIAN EMERALDS

Provenance determination for precious gems has been pursued for centuries using a variety of techniques, including variations in color, mineral inclusions and other phases, the concentration of major and trace elements, and spectroscopic data. This presentation reviews and compares: Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), X-ray Fluorescence (XRF), microscope identification of mineral inclusion and other phases, and Laser Induced Breakdown Spectroscopy (LIBS) as provenance determination methods for gemstone emerald, a variety of green beryl (Be₃Al₂Si₆O₁₈).

The Laser-Induced Breakdown Spectroscopy (LIBS) data presented in this study evaluates 500 emerald samples collected from 18 Colombian mines. Locations in this study include seven different mining areas: Chivor (3 mines), Cosquez (3 mines), Gachala (3 mines), La Pita (4 mines), Muzo (3 mines), Peñas Blancas (1 mine) and Ubala (1 mine).

Samples were analyzed using a Photon Machines Insight LIBS system with a Nd:YAG laser operating at 266 nm. Sixty spectra were acquired from each of the 500 samples in an argon environment. Using cluster analysis techniques of the LIBS data, positive identification of mine of origin for emeralds with success rate greater than 90% was achieved.

Laser-Induced Breakdown Spectroscopy is an appropriate approach for the emerald provenance problem, as well as other geochemical problems, as it can provide a statistically valid answer to the question of provenance determination. Previous techniques have only provided an indication of country of origin that requires a final decision to be made by a trained expert rather than an algorithm.