Abstract
A colour-changing garnet exhibits the “alexandrite effect”, whereby its colour changes from green in the presence of daylight to purplish red under incandescent light. This study examines this species of garnets as well as the causes of the colour change by using infrared and ultraviolet visible (UV-Vis) spectroscopy. The infrared spectra show that the colour-changing garnets in this paper belong to the solid solution of pyrope-spessartine type. CIE1931 XYZ colour matching functions are used to calculate the colour parameters influencing garnet colour-changing under different light sources. The UV-Vis spectra show two zones of transmittance, in the red region at 650–700 nm and the blue-green region at 460–510 nm. As they exhibit the same capacity to transmit light, the colour of the gem is determined by the external light source. The absorption bands of Cr3+ and V3+ at 574 nm in the UV-Vis spectra are the main cause of the change in colour. With the increase in the area of peak absorption, the differences in the chroma and colour of the garnet gradually increase in daylight and incandescent light, and it exhibits a more prominent colour-changing effect.
Highlights
Garnets are a common group of minerals that undergo complex changes in their composition
The island silicate garnet belongs to the cubic space group, Ia3d, Z = 8 [1], and has the general chemical formula A3B2[SiO4]3
The alexandrite effect is a phenomenon of distinctive changes in the colour of a gem when observed under daylight and incandescent light [13]
Summary
Garnets are a common group of minerals that undergo complex changes in their composition. A special kind of pyralspite garnet exhibits the alexandrite change in colour. Such garnets appear yellow-green in daylight and purple-red under incandescent light. The alexandrite effect is a phenomenon of distinctive changes in the colour of a gem when observed under daylight and incandescent light [13]. Guo et al [18,19] used Spectrophotometer Colour i5 and the GemDialogue colour chip to quantitatively characterize the colour of jadeite, and Minerals 2021, 11, 865. Guo et al [18,19] used Spectrophotom2eotfe1r5 Colour i5 and the GemDialogue colour chip to quantitatively characterize the colour of jadeite, and Tooms et al [20] used colour matching functions to calculate colour. The settings of the instrument were: resolution 4 cm−1; scanning range 400–2000 cm−1; run time 30 s per scan
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