Gypsum
ID: 68
Gypsum
Chemical Formula
CaSO₄·2H₂O
Hardness (Mohs)
2
Crystal System
Monoclinic
Streak
White
Luster
Vitreous, silky, pearly, dull
Diaphaneity
Transparent to translucent
Color
Colorless, white, gray, yellow, reddish, brown
Crystal Habit / Mode of Occurrence
Commonly tabular or prismatic crystals, often twinned (swallowtail twins are characteristic). Also found as fibrous (satin spar), massive granular (alabaster), and as large, clear crystals (selenite).
Fun Facts
Gypsum is a very soft mineral; you can scratch it easily with your fingernail! When heated, gypsum loses its water molecules and transforms into plaster of Paris.