Amethyst is a variety of bluish-purple quartz. It is a widespread mineral in nature, with larger specimens that can be cut and polished being used as gemstones, such crystals have been found in Brazil, Uruguay, Madagascar, Russia, Germany and Sri Lanka. Such large crystals are found in geodes (voids) of hydrothermal phyllodes associated with chalcedony and in volcanic rocks. The mineral frequently has a violet colour, varying shades to pink variants. The colour is unevenly distributed throughout the mass of the mineral, so streaks of darker shades appear. Determinants of colour are made by the presence of Fe4+ ions located in the atomic structure of the tetrahedral lattice. The colour of amethyst is thought to be due to the replacement of silicon atoms by iron atoms, a controversial theory.
Amethysts can be transformed by radiation into colourless crystals. A similar effect of colour change is achieved by exposing the mineral to sunlight or heat for a longer time. Amethyst is a prized gemstone and is frequently polished as a brilliant, navette or briolett. The dark purple variety is the most valuable, as is the ametrine variety a state of transition from amethyst to citrine. Manipulations by heating amethyst to 400 °C gives the yellow colour of citrine, similarly by heating can give the green colour of prasiolite, or perhaps colourless quartz, as can crystal degradation by cracking.