Pyrite
Sparta, Illinois
This specimen measures 7cm in diameter
Pyrite FeS2
Sulfide; Iron disulfide, often with substantial amounts of nickel and cobalt
Color and Luster: Pale yellow to brass-yellow, often tarnished with brown fild of iron oxide; sometimes iridescent; metalic; streak greenish black
Hardness: 6-6.5
Cleavage: None
Fracture: Uneven, brittle
Crystals: Isometeric; predominantly in cubes adn pyritohedrons with parallel striations of faces, less commonly in octahedrons; also in nodules adn massive forms; fint to coarse granular, fibrous, mammillary, stalactitic
Photo by Mary Sutherland
Descriptions taken from
The Audubon Society Field Guide to North American Rocks and Minerals