Nondestructive identification of emeralds using inclusion morphology and chemical composition analyses

Emeralds are crucial evidentiary materials in cases such as gem fraud, and highly accurate, nondestructive methods for gemstone discrimination are therefore in high demand in forensic investigations. In this study, we aimed to establish a nondestructive authentication protocol based on the inclusion morphology and chemical composition of natural and flux-synthesized emeralds. As a fundamental examination, imaging analyses using micro-X-ray fluorescence spectrometry and Raman spectroscopy, together with localized synchrotron radiation X-ray diffraction analysis, were conducted on thin-sectioned samples. Inclusions observed under an optical microscope were classified into six types, and most of the constituent components, including biotite, SiO₂, Be₂SiO₄, MoO₃, and polycrystalline emerald, were identified and correlated with their morphological features. Based on the findings obtained from the fundamental examinations, practical nondestructive examinations were performed using portable energy-dispersive X-ray fluorescence spectrometry and optical microscopy. To minimize subjectivity, as the first step of the discrimination protocol, principal component analysis (PCA) and hierarchical cluster analysis (HCA) were performed using the matrix X-ray intensity ratios of Al/Si, Cr/Al, and Cr/Si. By applying a four-step approach consisting of (1) exploratory PCA and HCA, (2) comparison of matrix X-ray intensity ratios, (3) comparison of inclusion- and flux-derived elements, and (4) classification of inclusion morphology, reliable discrimination was achieved. This study advances forensic analysis of gemstone-related trace evidence and extends to the forensic discrimination and authentication of gemstones beyond emeralds.