Amethyst is a form of silica mineral quartz that is often valued as a semiprecious gem for its violet-to-purple color and attractively shaped crystals. Many experts attribute this unique coloration to small amounts of iron impurities in the crystal lattice of quartz, which usually exhibits a clear and colorless texture. Other scientists, however, theorize that the mineral's color arises from the presence of minute quantities of manganese or hydrocarbons.

Amethysts can occur as long prismatic crystals that have a hexagonal pyramid shape at either end, or as crystalline crusts lining internal cavities of rocks. The amethyst-containing minerals often lose their color or acquire a yellowish tint when heated. Citrine gemstones, which are found somewhat rarely in nature, are often created by heat-treatment of low grade, inexpensive amethysts.

Although this gemstone is found all over the world, different localities can produce amethysts that are unique to a particular region or even to a particular mine. Experts are often able to identify the source mine for an amethyst mineral formation by examining features such as color, crystalline geometry, and inclusions. The finest specimens are usually found in India, Sri Lanka, and Brazil. Amethyst crystals also occur from the Ural Mountains of Russia and Europe to many parts of Scotland, and in areas of North America, such as Ontario and North Carolina.

The name amethyst derives from the Greek root amethystos, "not intoxicated," which some speculate expresses the ancient folk belief that the stone protects its owner against drunkenness. More likely, the ancient Greeks were probably referring to the wine-like color of some stones.

A majority of gem scholars agree that the tradition of birthstones arose from the Breastplate of Aaron described in the Bible (Exodus 28, 15-30). The breastplate was a ceremonial religious garment set with twelve gemstones that represented the twelve tribes of Israel and corresponded with the twelve signs of the zodiac and the twelve months of the year. There are many different birthstone lists, however, and some argue that they should be assigned by astrological sign and not month. Amethyst is the birthstone for February and the zodiac sign Aquarius.