Cuprite. Image source: Mineralogical Information Institute.
Cuprite is a reddish-brown, brittle, translucent mineral consisting of copper (I) oxide (Cu2O); a major ore of copper. Its octahedral, dodecahedral, and cubic crystals occur in the cubic system, and it also occurs in grains. It is formed by oxidation of copper sulfide ores and so is commonly found near the surface. Cuprite is found in Europe, Australia, Arizona, southwest Africa, and South America. Hardness 3.5–4; relative density 6.1.