A Study on Colour-Change Effect and Colour Quantification of Rare Earth Minerals and Gem Materials

Some REE-coloured minerals exhibit strong colour-change effect. Using electron probe microanalysis (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and UV-Vis spectra analysis, this study systematically investigates the effects of light sources with different emission mechanisms and colour temperatures on the colour-change effect of natural and synthetic REE-coloured mineral samples, including shomiokite-(Y), xenotime-(Y), bastnäsite-(Ce), and Nd-doped synthetic fluorite. The results show that both shomiokite-(Y) and xenotime-(Y) are mainly coloured by Er³⁺, while Nd³⁺, Sm³⁺, Dy³⁺ and Tm³⁺ also contribute to the colouration. The bastnäsite-(Ce) is coloured by Nd³⁺ and Pr³⁺. Based on colour calculations in the CIE 1976 L^*a^*b^* colour space and comparisons with actual photographs, this study demonstrates that, among the investigated light sources, the light emission mechanism has a significant impact on the colour presentation of mineral samples, while changes in colour temperature within the same emission mechanism have relatively limited effects. Under fluorescent lamps, shomiokite-(Y) exhibits a pinkish hue, whereas under LED lamps, it appears pale orange-yellow. Xenotime-(Y) shows a pinkish-orange colour under white fluorescent lamps and yellow under white LED lamps. Bastnäsite-(Ce) appears orange under fluorescent lamps and yellow under LED lamps. Nd-doped synthetic fluorite is nearly colourless under fluorescent lamps, but displays a purple colour under LED lamps. Analysis results of light sources used in quality supervision and inspection institutions and rules in relevant national standards indicate that the regulations for light sources in gemstone grading standards are relatively vague, and differences in the spectral characteristics of light sources may lead to inconsistencies in colour evaluation when grading. Therefore, a greater emphasis should be placed on the selection and specification of illuminants in relevant national standards. This study aims to provide a scientific methodological basis for the selection and evaluation of light sources.