Crystalline quartz in shades of purple, lilac, or mauve is called amethyst, a stone traditionally worn to guard against drunkenness and to instil a sober and serious mind. Amethyst is dichroic, showing a bluish or reddish purple tinge when viewed from different angles. Usually faceted as a mixed or step cut, amethyst has distinctive inclusions that look like tiger stripes, thumbprints or feathers. Some amethyst is heat treated to change the colour to yellow, producing citrine. Crystals that are part citrine and part amethyst are called ametrine.
Occurrence - Amethyst is found in alluvial deposits or in geodes. Some of the largest geodes containing amethyst are in Brazil. Amethyst from the Urals (Russia) has a reddish tinge; Canadian amethyst is violet. Other localities include Sri Lanka, India, Uruguay, Madagascar and the USA, Germany, Australia, Namibia and Zambia.
Sources: Hall C (2002). Gemstones.United States. Smithsonian Handbooks