1. A mineral of a white or a slightly bluish color, usually massive.

It is anhydrous sulphate of lime, and differs from gypsum because it does not contain water (whence the name).

2. A mineral consisting of anhydrous calcium sulfate, commonly formed as massive or laminated evaporites; which may be altered by hydration to gypsum, as in weathering.

A compact, granular sedimentary rock harder than gypsum, composed essentially of calcium sulfate, resembling marble and deposited by evaporation.