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From The Collaborative International Dictionary of English v.0.48:
Gypsum \Gyp"sum\ (j[i^]p"s[u^]m), n. [L. gypsum, Gr. gy`psos; cf. Ar. jibs plaster, mortar, Per. jabs[imac]n lime.] (Min.) 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. [1913 Webster]