Iolite is the gem variety of the mineral Cordierite. When the mineral is transparent and of gem quality, it is known as "iolite" in the gem. It describes the transparent to the translucent form of Cordierite, which has an appearance similar to sapphire and tanzanite. Cordierite is a silicate mineral that is found in metamorphic and igneous rocks.
Iolite can be light to deep blue, and usually has a purplish tinge to it and is one of the most strongly pleochroic minerals. Usually transparent, iolite sometimes contains inclusions that cause attractive phenomena. Iolite is strongly pleochroic and will exhibit a different color when viewed at different angles. It is the most pleochroic gemstone, with this effect being noticeable on all transparent Iolite gemstones. A blue or violet-blue Iolite gemstone will become a duller gray or grayish-yellow when viewed at a different angle.
It is much softer than sapphires and is abundantly found in Australia (Northern Territory), Brazil, Burma, Canada (Yellowknife area of the Northwest Territories), India, Madagascar, Namibia, Sri Lanka, Tanzania, and the United States (Connecticut).
Cordierite, which was discovered in 1813, in specimens from Níjar, Almería (Spain) is named after the French geologist Louis Cordier from the Greek word for "violet", alluding to the purplish tinge often exhibited by this gemstone. The name "iolite" comes from the Greek word for violet. Another old name is dichroite, a Greek word meaning "two-colored rock", a reference to cordierite's strong pleochroism. It has also been called "water-sapphire" and "Vikings' Compass" because of its usefulness in determining the direction of the sun on overcast days, the Vikings have used it for this purpose.
Iolite is the gemstone for the 21st wedding anniversary.