Identification Methods of Colourless Natural and Synthetic Sapphires

Colourless natural sapphire, known for its bright luster and high transparency, is popular in the gemstone market. However, colourless synthetic sapphires are frequently sold as natural sapphires, and internally clean stones are difficult to distinguish natural from synthetic ones using conventional methods. In this study, systematic comparative investigation of colourless natural and synthetic sapphire samples is conducted using the conventional gemmological tests, energy-dispersive X-ray fluorescence spectrometry (EDXRF), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The results show that approximately 70% of the colourless natural sapphire samples exhibit weak to strong red, orange or blue-white fluorescence under long-wave ultraviolet light (365 nm), with common inclusions such as healed fissures and small crystals. In contrast, all colourless synthetic sapphire samples are non-fluorescent and rarely contain inclusions. LA-ICP-MS results reveal significant differences in both the concentrations and types of trace elements between colourless natural and synthetic sapphires, with Ga and Fe being the most discriminative elements. Natural samples contain 10.0-141.6 μg/g Ga and 109.0-2 225.0 μg/g Fe elements, whereas both Ga and Fe in synthetic samples are below the detection limit. Raman spectraanalysis indicates that both natural and synthetic samples display a scattering peak near 417 cm^-1, but the Full Width at Half Maximum (FWHM) of this peak differs markedly: the FWHM of synthetic samples are all ≤ 2.35 cm^-1, while those of natural samples are all ≥ 2.42 cm^-1. Under 532 nm excitation, natural and synthetic samples exhibit photoluminescence peaks at 693.3 nm and 694.7 nm with identical peak positions, but the photoluminescence intensity of synthetic samples is consistently lower than that of natural ones. Based on these findings, an identification protocol is proposed that integrates conventional instruments for preliminary screening, EDXRF and LA-ICP-MS trace element testing as the key technique, and Raman spectroscopy and photoluminescence (PL) intensity as auxiliary indicators, providing a systematic reference for the accurate identification of colourless sapphires.