Influence of Heat Treatment on Colour of Amethyst and Its Spectral Characteristic

Some salesmen often sell the heat treated amethysts for imitating the citrines in order to improve the price. In this paper, the amethyst samples with different purple were heated. The colour and spectral characteristics of amethyst samples before and after the heat treatment were studied by using the gem microscope, FTIR and UV-Vis spectrum. The authors found that the colour of amethyst samples became deeper and deeper when the temperature increased, which changed from purple to colourless to yellow at the temperature range of 400-500℃. And this colour-change temperature was related with holding time. The infrared spectrum of the amethyst after heat treatment with yellow colour had the absorption peaks at 3 854、3 738、3 585、3 436、2 675、2 362 and 2 233 cm^-1. But natural citrine haven't the absorption peak at 3 585 cm^-1. The intensity of UV-Vis absorption peak at 540 nm in amethyst sample, decreased when the temperature increased.