AZURITE (Copper Carbonate) crystal from M,Cissi, Er Rachidia, Morocco. A large vein of drusy azurite crystals.
Copper (Cu) is one of a few elements occurring natively as a mineral. It has a specific gravity of 8.9 and a hardness of 2.5-3. It is reddish, orangish, and/or brown in colour when fresh, but usually forms a black or green tarnish in air. The green tarnish may be called patina or vertigris and is in fact copper (ii) carbonate.
Copper was one of the first metals to be used by humanity, predated only by gold (another native element) and iron from meteors. While humanity may have been first introduced to copper as a native element, the majority of copper we use is from copper sulphides (in particular, chalcopyrite and less frequently chalcocite), as deposits of those minerals are far more abundant than native copper deposits. In fact, native copper deposits are so depleted their only real use is as mineralogical specimens or ornamental pieces.
Native copper usually occurs with sulphide copper deposits, in basalts, sedimentary rocks, and very occasionally in meteorites. It forms crystals, which may be cubic, dodecahedra, or tetrahexahedra, but it is more common to see native copper in aborescent, branching, or wiry habits. Copper is malleable (can be hammered flat without breaking), ductile (can be drawn into a wire), and sectile (can be cut into pieces).
Interestingly, copper is biostatic, meaning that bacteria cannot grow on it. It is also generally antimicrobial and it and a number of its alloys can kill a number of viruses and funguses in addition to a large amount of bacterial species. When applied to the hull of a ship, it prevents marine organisms such as barnacles and mussels from attaching to the hull.
Image 1: Native Copper on calcite by Parent Gery. Locality not given. Public domain, retrieved from Commons.
Image 2: Native Copper from Lake Superior, Michigan, USA, by Didier Descouens. Via Wikimedia Commons.