Genesis and Influencing Factors of the Colour of Chrysoprase: Colourimetric Analysis Based on the Cie 1976 L*a*b* Uniform Colour Space

Abstract

The colour of chrysoprase is produced by Ni cations and influenced by other transition-metal cations and crystallinity. Presented here is a study of the origin of chrysoprase colour and an exploration of the factors that influence it from the perspective of gemstone chromaticity. Transmission electron microscopy and X-ray fluorescence, ultraviolet–visible and Raman spectroscopies are used to investigate 41 gem-quality chrysoprase samples, and the results show that chrysoprase colour results from nanometre-size inclusions of pimelite. Under a 6504-K fluorescent lamp, chrysoprase colour is divided into the grades of fancy, fancy intense and fancy deep. The lightness of chrysoprase is affected mainly by its Cr content, the chroma is affected by its Ni content and the hue angle is affected by the sum of its Cr and Fe contents. Chrysoprase hue and chroma are related significantly to the transmission window that occurs between the two main bands centred at 380 and 660 nm and the absorption peak at 660 nm in the ultraviolet–visible spectrum. Chrysoprase with low crystallinity has more Ni and a higher chroma.