High-spatial-resolution oxygen isotopic analysis to distinguish natural from synthetic corundum

Abstract. Gem-quality corundum varieties of ruby and sapphire are one of the most valuable and desired gemstones. Due to their rarity, new methods of synthesis and treatment were developed over the last decades, complicating the reliable identification between natural, treated, and synthetic specimens. Among the geochemical methods used for identification, trace element analysis using laser ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS) is widely applied. However, solely relying on LA-ICP-MS trace element analysis for differentiation between natural and synthetic corundum origins, especially when grown by the hydrothermal method, can potentially lead to misidentifications. To further enable geochemical tracing of corundum, this study explores secondary ion mass spectrometry (SIMS) oxygen isotope analysis. High-spatial-resolution SIMS δ18O analysis of hydrothermally synthesized corundum yielded values between -7.84±0.13 ‰ and -14.54±0.13 ‰ (relative to Vienna standard mean ocean water, VSMOW; 1 standard error) that are atypical for natural corundum. For flame fusion corundum, SIMS δ18O analyses are in the range of -6.73±0.13 ‰ to -17.46±0.13 ‰ for sapphires of blue, yellow, and orange colour and +28.51±0.11 ‰ to +30.47±0.10 ‰ for ruby, which, in both cases, are again atypical for natural corundum. SIMS δ18O analysis of corundum thus has strong potential to distinguish synthetic and natural corundum.