High-throughput identification of geographical origins of rubies using hyperspectral visible and fluorescence spectroscopy

Abstract

This study explored hyperspectral (HS) visible and fluorescence spectroscopy as a high-throughput analytical tool for discrimination of rubies originating from Myanmar (MM), Mozambique (MZ), and Thailand (TH). Rubies are a variety of the mineral corundum (Al2O3), and their color is mainly related to Cr and Fe. In this study, multiple rubies were loaded onto a carbon-coated plate and measured under a HS camera for high-throughput analysis. The absorption peaks around 420 nm and 558 nm were apparent in the visible spectra. Interestingly, a Raman peak at 692 nm also appeared due to excitation by ultraviolet radiation emitted from a visible lamp. However, due to various factors such as surface reflection, inclusions, and impurities, baseline fluctuations among the visible spectra were large. In contrast, the fluorescence spectra were more reproducible, and the emission peaks of rubies were better discernible. Therefore, support vector machine (SVM)-based discrimination accuracy using the fluorescence spectra was 97.8 % based on 100 repetitions of 5-fold cross-validation, higher than 95.8 % employing the visible spectra, and slightly higher or comparable to that (96.6 %) based on the elemental analysis using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Therefore, the demonstrated HS fluorescence scheme, which enables high-throughput analysis of multiple rubies, could be a versatile alternative in the field of gemstone evaluation where individual sample-by-sample measurement has been a common practice.