Image source: Mineralogical Information Institute.
A dark gray or black mineral, copper (I) sulfide (Cu2S). Chalcocite is a major copper ore found mainly in sulfur deposits. The crystals occur in orthorhombic granular masses, or rarely in prismatic form. It is found (often with bornite) in the Ural Mountains, Africa, South America, Alaska, Connecticut, and the southern United States. Hardness 2.5 to 3.0; relative density 5.7.