Gemmological and Spectroscopic Characteristics of Colour-Change Andradite

Gemstones exhibiting a colour-change effect, which display different colours under varying light sources, generally possess higher aesthetic appeal and market value compared to their non-colour-changing counterparts. In this study, a combination of techniques including conventional gemmological test, electron probe microanalysis (EPMA), laser ablation inductively coupled plasma mass spectrome-try (LA-ICP-MS), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and ultraviolet-visible absorption spectroscopy (UV-Vis) were employed to analyze the chemical composition and colour genesis of 7 colour-change garnet samples. The studied samples are andradite (And ≥ 90%) containing very low levels of Ti, Al, Cr, Mn, and Mg (< 0.2%). In most samples, the concentrations of Cr³⁺ and V³⁺ are below 4×10^-6. Three absorption peaks near 578, 602 nm and 613 nm, caused by Fe²⁺, Cr³⁺ and V³⁺ absorption are superimposed to form a broad absorption band centered at 613 nm, resulting in the formation of 2 transmission windows centered at 552 nm and above 650 nm. These transmission windows allow the transmission of yellowish-green and orange-red light, which is key to the colour-change effect in andradite.