Gypsum
Chihuahua, Mexico
This specimen measures 13cm x 13cm
Gypsum CaSO4.2H2O
Sulfates; hydrous calcium sulfate
Color and Luster: White, colorless, gray, yellow, red, brown; vitreous, pearly on cleavage surfaces; streak white
Hardness: 1.5-2
Cleavage: Perfect in one direction, distinct in two others
Fracture: Conchoidal, splintery, brittle; thin flakes flexible
Crystals: Monoclinic; untwinned single crystals showing rhombic form common; may be twinned as arrowhead shape with beveled edges; also granular, massive, fibrous(Var. satin spar), earthy
Photo by Lance Kearns
Descriptions taken from
The Audubon Society Field Guide to North American Rocks and Minerals