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Measurement of Gem Colour Using a Computer Vision System: A Case Study with Jadeite-Jade

Explaining Colour Change in Pyrope-Spessartine Garnets

by and *
School of Gemmology, China University of Geosciences (Beijing), Beijing 100083, China
Author to whom correspondence should be addressed.
Academic Editors: Lluís Casas and Roberta Di Febo
Minerals 2021, 11(8), 865;
Received: 18 June 2021 / Revised: 3 August 2021 / Accepted: 6 August 2021 / Published: 11 August 2021
(This article belongs to the Special Issue Colours in Minerals and Rocks)
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. View Full-Text
Keywords: colour-changing garnet; FTIR; UV-Vis spectra; CIE1931 XYZ colour matching functions colour-changing garnet; FTIR; UV-Vis spectra; CIE1931 XYZ colour matching functions
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MDPI and ACS Style

Qiu, Y.; Guo, Y. Explaining Colour Change in Pyrope-Spessartine Garnets. Minerals 2021, 11, 865.

AMA Style

Qiu Y, Guo Y. Explaining Colour Change in Pyrope-Spessartine Garnets. Minerals. 2021; 11(8):865.

Chicago/Turabian Style

Qiu, Yan, and Ying Guo. 2021. "Explaining Colour Change in Pyrope-Spessartine Garnets" Minerals 11, no. 8: 865.

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