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]

The Collaborative International Dictionary of English v.0.48:

Selenite \Sel"e*nite\, n. (Chem.)
   A salt of selenious acid.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Selenite \Sel"e*nite\, n. [L. selenites, Gr. ???? (sc. ???),
   from selh`nh the moon. So called from a fancied resemblance
   in luster or appearance to the moon.] (Min.)
   A variety of gypsum, occuring in transparent crystals or
   crystalline masses.
   [1913 Webster] Selenitic