In its purest form, quartz is colourless and transparent. Colouring is due to impurities and can be the result of proximity over millions of years to radioactive rocks. Smoky quartz contains aluminium impurities, but its colour disappears above 300–400°C (∼570–670 K). The colours vary with the impurities, as in violet amethyst, which is merely an irradiated quartz with iron oxide impurities (Fe4+); when heated, the iron impurities change their oxidation state (Fe3+) and yellow citrine is obtained. Ametrine is a mix of citrine and amethyst. Source: Voyage dans le cristal. Photographs: quartz rock crystal (La Gardette, Isère, France) and amethyst (Montana, USA) (copyright Collection des Muséum de Grenoble); smoky quartz (Curie family, copyright Collection des Minéraux de Jussieu, UPMC, Paris); ametrine (copyright Eddy-Vleeschdrager).