Gypsum
Gypsum (jip"sum) , noun
[Latin gypsum, Greek gy`psos; compare Arabic jibs plaster, mortar, Per. jabsīn lime.]
(Mineralogy) A mineral consisting of the hydrous sulphate of lime (calcium). When calcined, it forms plaster of Paris. Selenite is a transparent, crystalline variety; alabaster, a fine, white, massive variety.