Amethyst is a well known mineral and gemstone. It is the purple variety of the mineral Quartz, and its most 
valuable and prized variety. Its name derives from the Greek "amethystos", which means "not drunken", as Amethyst in antiquity was thought to ward off drunkenness.
The color of some Amethyst specimens from certain localities slowly fade upon prolonged exposure to light. When used as a gemstone, Amethyst is often heat treated to deepen the color, or to transform it into "Citrine". Some varieties may also change to a light green color, which is given the trade name "Prasiolite", or "Green Amethyst", as it is more commonly known in the gem trade.
Article supplied by Minerals.net