Amethyst is a variety of violet-blue quartz. Amethyst is a very widespread mineral in nature; larger specimens that can be cut and polished are 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 vents associated with chalcedony and in volcanic rocks.
The mineral is frequently purple in color, varying in shades to shades of pink. The color is unevenly distributed in the mass of the mineral, so streaks of darker shades appear. Color determinants are determined by the presence of Fe4+ ions placed in the atomic structure of the tetrahedral network. The color of amethyst is said to be due to the replacement of silicon atoms with iron atoms; this is a controversial theory. Amethysts can be transformed by exposure to radiation into colorless crystals. A similar effect of color change is achieved by exposing the mineral to the sun or heat for a longer time.
Amethyst is a prized gemstone, frequently being cut as a brilliant, shuttle or briolette. The dark purple variety is the most valuable, as is the ametrine variety, a transitional state from amethyst to citrine. Manipulations by heating amethyst to 400 °C produce the yellow color of citrine; similarly, heating can produce the green color of prasiolite, or perhaps colorless quartz, as well as the degradation of the crystal by cracking.