Calcite
Smith Quarry
Black Rock, Arkansas
The largest crystal measures 12cm by 6cm
Calcite CaCO3
Carbonate; calcium carbonate
Color and Luster: White, colorless, pale shades of gray, yellow, red, green, blue, brown to black when impure; vitreous, dull; streak white
Hardness: 3
Cleavage: Perfect in 3 directions, forming a rhombohedron
Fracture: Conchoidal, seldom observed
Crystals: Hexagonal; crystals commonly showing a rhombohedron, scalenohedron, and prism; also tabular, acicular, cleavable, granular, compact, stalactitic, oolitic, earthy; twinned crystals very common
Photo by Mary Sutherland
Descriptions taken from
The Audubon Society Field Guide to North American Rocks and Minerals