Selenite is a crystalline variety of gypsum, and very often confused with its close relative Satin Spar. Selenite forms in clear 'window' plates, whereas Satin Spar forms in white fibrous crystals that display a Cat's Eye effect. Selenite is named after the Greek Goddess of the Moon, Selene, and it was believed that in the ancient world that certain crystals followed the waxing and waning of the moon, including Selenite. Selenite and Satin Spar have been used for centuries in cleansing and protection grids and rituals - the ancient Mesopotamians described in great detail how to use these crystals to ward away evil Spirits from sick people. The clear plates of Selenite were also used for windows by the ancient Greeks and Romans, and the church of Santa Sabina in Rome still has its original Selenite windows.