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 6504 K 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.

The Beihong agate from the southern section of the Xing’an Mountains in northeastern China is a kind of cryptocrystalline agate with a yellow to orange-red colour. However, it has been less studied in previous research, and there is a lack of quantitative study on the cause of its colour. In this study, the colour of Beihong agate was quantified by a colourimeter, and the quantitative relationships between colour and spectral characteristics of Beihong agate were studied by XRF, Raman spectra, UV-VIS spectra, and a heat treatment experiment. The results show a high correlation between the lightness and the hue angle of the Beihong agate. The change of total Fe content can significantly affect the lightness and the hue of the Beihong agate. The first derivative curve can effectively distinguish the relative contents of goethite and hematite in the Beihong agate, and the position of a primary trough is related significantly to the colour of the Beihong agate.