Block of Selenite
Selenite was named after Selene the Greek goddess of the Moon, meaning literally the stone of the Moon (not to be confused with albite-orthoclase feldspar moonstone). The name alludes to the moon-like pale reflection the stone can give. Transparent to translucent with a glowing transparency, throughout history it has been compared to liquid light.
In the middle ages it was used as a substitute for glass.