Chalcocite

Chalcocite, mineral consisting of copper sulfide, with the formula Cu₂S. It crystallizes in the orthorhombic system (see Crystal) and has a hardness from 2.5 to 3 and a specific gravity from 5.5 to 5.8. An important ore of copper, it is frequently found in granular or compact masses. The luster is metallic and the color lead gray, which tarnishes to dull black on exposure to air. Chalcocite is usually found in nature in association with other copper minerals, as in the Cornwall mines in England, which have beautiful crystal specimens. It also occurs in southwest Africa, Mexico, and South America and in the United States in Bristol, Connecticut, and in the Rocky Mountains.