Selenite (chemical formula: CaSO₄·2H₂O) is a hydrous calcium sulfate, meaning it is composed of oxygen, sulfur, hydrogen, calcium and water. It is basically a glassy, well-crystallized form of gypsum and is often referred to as satin spar.

Selenite is soft, and can easily be scratched with a fingernail. The thin crystals can be slightly flexible, but will snap if bent too far. Some crystals found in Kentucky have grown in severely curved patterns and flower-like petals.

In some very rare instances, water becomes encased in a cavity as the crystal forms. In these samples, the tiny bubble of water can be seen moving as you rotate the stone.

In dry desert conditions, sand may become trapped inside the crystals as they form. Sandy selenite formations can take on the shape of an hourglass or the more familiar "desert rose" shape. These selenite crystals are totally opaque and have been found in large quantities in Oklahoma, Texas and the Sahara Desert.

Selenite crystals are often found in the Gault Clay and London Clay formations of England.