Copper, as native copper, is one of the few metallic elements to occur in uncombined form as a natural mineral, although most commonly occurs in oxidized states and mixed with other elements. Native copper was an important ore of copper in historic times and was utilized by pre-historic peoples.
Native copper occurs as rarely isometric cubic and octahedral crystals, but more typically as irregular masses and fracture fillings. It is a copper-red color on fresh surfaces, but typically is weathered and coated with a green tarnish of copper(II) carbonate. Its specific gravity is 8.9 and the hardness is 2.5.
The mines of the Keweenaw native copper deposits of Upper Michigan and adjacent northern Wisconsin were the major copper producers of the 19th and early 20th centuries, and are the largest deposits of native copper in the world. The first mine in the Keweenaw Peninsula (which is nicknamed "Copper Country") opened in 1771. Evidence exists of copper trading routes occurring throughout North America amongst the Native Aboriginal peoples, proven by isotopic analysis.
The name copper comes from the Greek kyprios, of Cyprus, the location of pre-historic copper mines.