Selenite gets its name from Selene, the Greek Moon, and every crystal shop sells it as the lunar stone. It is gypsum: hydrated calcium sulfate, soft enough to scratch with a fingernail.

The name is about the pearly sheen rather than any connection to the Moon, and mineralogists apply it to a specific transparent variety. Modern practice uses it for cleansing other crystals and for charging under a full moon. Worth knowing that it dissolves in water, so the one thing you should not do with it is the moon-water jar.