Abstract
Chrysoprase is a popular gemstone with consumers because of its charming apple green colour but a scientific classification of its colour has not yet been achieved. In this research, we determined the most effective background of the Munsell Chart for chrysoprase colour grading under a 6504K fluorescent lamp and applied an affinity propagation (AP) clustering algorithm to the colour grading of coloured gems for the first time. Forty gem-quality chrysoprase samples from Australia were studied using a UV-VIS spectrophotometer and Munsell neutral grey backgrounds. The results determined the effects of a Munsell neutral grey background on the observed colour. It was found that the Munsell N9.5 background was the most effective for colour grading in this case. The observed chrysoprase colours were classified into five groups: Fancy Light, Fancy, Fancy Intense, Fancy Deep and Fancy Dark. The feasibility of the colour grading scheme was verified using the colour difference formula DE2000.
Highlights
Chrysoprase is an apple green fine crystalline variety of polycrystalline quartz
One hypothesis associates the colour of chrysoprase with the presence of fine dispersed nickel-bearing layered silicate minerals such as willemseite [1], kerolite [2,3] and pimelite [4,5], which usually occur in the chrysoprase deposits
An alternative hypothesis is that the colour is due to the presence of evenly distributed bunsenite (NiO) [6,7]. The evidence for this phase comes from the weak reflection of the X-ray pattern of Kazakhstan chrysoprase at 2.39 Å [7]
Summary
Chrysoprase is an apple green fine crystalline variety of polycrystalline quartz. It is found all over the world but the best quality gems come from Australia. An alternative hypothesis is that the colour is due to the presence of evenly distributed bunsenite (NiO) [6,7]. The evidence for this phase comes from the weak reflection of the X-ray pattern of Kazakhstan chrysoprase at 2.39 Å [7]
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