Named after the greek word for the moon, Selenite crystals compose one of the largest crystals in the world. Picture is from Chihuahua, Mexico. The largest of those crystals weighs 55 tonnes, is 33 feet long and is over half a million years old. Selenite is a form of Gypsum which is an evaporative mineral, frequently found in alkaline lake muds, clay beds, evaporated seas, salt flats, salt springs and caves.