The water-clear quartz known as rock crystal was believed by the ancient Greeks to be permanently frozen water. Magicians through the centuries used it as crystal balls to contemplate the future.
Color variations of quartz depend on the oxidation state of traces of iron. Amethyst ranges from pale purple to dark violet, with color proportional to iron content and often with alternate triangular sectors of purple and white due to twinning. Named for the Greek word meaning “not drunken,” the stone was thought to protect the wearer from effects of alcohol. Mythology also identifies amethyst as warding off harm in battle.
Smoky quartz varies from pale brown to almost black. Known confusingly as smoky topaz, smoky quartz, or cairngorm is the “national gem” of Scotland. Large crystals of rich color occur in the Cairngorm Mountains.
Citrine varies from a light golden yellow to orange-yellow. Natural citrine is rare, and most yellow quartz is actually heat-treated amethyst or smoky quartz.
Rose quartz, colored by titanium or manganese, is usually translucent and milky. Crystals are very unusual; it occurs most commonly in the massive form.
Quartz often grows over other minerals. Typical inclusions are rutile, tourmaline, mica, and acicular or fibrous crystals of an amphibole. Sagenite or rutilated quartz is rock crystal containing fine needlelike crystals of rutile.
Quartz is a very common mineral and occurs in many igneous rocks, is common in metamorphic rocks, and makes up sandstones and gravels. Most gem quartz occurs in vein deposits or as geodes.