Gypsum (also, var Selenite)

Chemical Formula : CaSO₄-2H₂O
Hardness : 2
Cleavage : Three, one perfect, two distinct
Luster : Vitreous, pearly, or silky
Color : Colorless, also white, gray, yellowish, or orange-brown when massive.
Streak : White
Density : 2.32
Diagnostic Features and Trivia : Gypsum is the definition of hardness 2 on Moh's scale. Selenite is a form of gypsum with beautiful, clear crystals.
Crystal System and Habit : Monoclinic; commonly tabular on {010}.
Twinning : Conatact twinning on {100} or {-101} producing swallow-tail twins.
Polymorphs : None
Index of Refraction : alpha=1.519-1.521; beta=1.522-1.526; gamma=1.529-1.531