Amethyst is a violet variety of QUARTZ, and has a chemical composition of SiO2. Gem quality amethyst is a relatively inexpensive gemstone often used in jewelry. Amethyst mineral specimens, geodes and large cathedrals are prized by collectors. Amethyst occurs in a wide range of purple shades from pale lilac to deep purple. When amethyst is heated, it becomes CITRINE, a yellow or orange variety of quartz.