The effect of heat treatment on amethyst color was studied from a new perspective of chromaticity of gemstones and the cause of amethyst coloration was discussed based on the results of X-ray diffraction, ultraviolet–visible spectroscopy. The results show that the amethyst color has no significant relationship with cell parameters but the crystallinity index decreases as temperature rises. The absorption band at 545 nm in the UV–visible spectrum can be related to a charge-transfer transition of Fe³⁺ and O²⁻, which has a significant relationship with amethyst lightness and chroma. The color at different temperatures can be divided into three stages: The amethyst stage with temperature below 420 °C, the prasiolite stage with temperature between 420 and 440 °C where the color center is the most unstable, the citrine stage with temperature above 440 °C. The color change degree of heated amethyst is related to its initial color. When the initial color is darker, the color difference of heated amethyst is larger, and the easier it is to change the color after heat treatment. A more appropriate heating temperature to obtain citrine by heating amethyst is about 560 °C.

Peridot’s yellowish-green color is not only produced by ${{\rm Fe}^{2+}}$ but is also influenced by background, observer, illuminance, and color temperature of the light source. In this research, we studied the most effective background for peridot color grading under a 6504K fluorescent lamp. Ninety-five gem-quality peridot samples from China, Pakistan, and Myanmar were studied using electron probe micro-analyzer, spectrophotometer, Munsell neutral gray backgrounds, and standard gold alloy backgrounds. The results provided the color effects of 24K, 22K, 20K, 18K, and 16K gold alloy backgrounds and Munsell neutral gray backgrounds on the peridot’s color. It was found that the Munsell N9 background seems to be the most effective background for peridot color grading. Using the Munsell N9 background, peridot colors were graded into three levels from good to bad: Fancy Intense, Fancy Deep, and Fancy by using K-means clustering analysis and Fisher discriminant analysis methods. The peridot’s color level reverse phenomenon should be considered when the background varies between Munsell N7 and N9 gray.

The genesis and influencing factors of the colour of chrysoprase were studied based on the results of transmission electron microscopy and X-ray fluorescence, ultraviolet–visible and Raman spectroscopies. The results show that 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 chromium content, the chroma is affected by its nickel content and the hue angle is affected by the sum of its chromium and ferrum contents. The colour of chrysoprase is related significantly to the transmission window that occurs between the two main bands centred at 380 and 660 nm and the absorption peaks at 380 and 660 nm in the ultraviolet–visible spectrum. Chrysoprase with low crystallinity has more nickel and a higher chroma. The inclusions that cause the chrysoprase colour have been identified as pimelite.

With the help of irradiation, [FeO4]4- was excited by one electron from ferric electronic shell, it absorbed the visible light of 550nm, and then made crystal appears golden color which named citrine.27 pieces citrine was confirmed as natural from 32 pieces golden citrine by IR technology, their color parameters were calculated by the colorimeter tests, then how could hue, lightness and chroma contribute to visual color were analyzed too.It reveals that chroma of natural citrines are negatively correlated with hue and lightness, by the control of hue angle on the impact of the chroma, the co-effect of hue and lightness of natural citrine produce moderate influence on chroma. Without the consideration of chroma influence on lightness, lightness and hue angle are high correlated (r=0.924), it is tested the high linear correlation of citrine lightness and hue; while with the consideration of chroma influence on lightness, lightness and hue angle also high correlated (r=0.897), and the high linear correlation of citrine lightness and hue is confirmed. It draws conclusion that in the limit the citrine golden, lightness should be regarded as the first factor when citrine quality evaluation was taken, with the most intuitive representative and most consistent with the expression of perception effect, then the effect of hue and chroma was considered furthermore.