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 which look like tiger-stripes, thumb-prints, or feathers. Some amethyst is heat-treated to change the color to yellow, producing citrine (see opposite). Crystals that are part citrine.

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.

Poor-quality material is often tumbled to make beads. If a stone is pale it may be set in a closed setting or have foil placed behind it to enhance the color. Amethyst has been imitated by glass and synthetic corundum .