Native copper

Lore & Background

Native copper is a naturally occurring, uncombined form of the metallic element copper, one of the few metals found in nature in its pure, elemental state rather than as a compound. It typically appears as irregular masses and fracture fillings within rock, and only rarely forms well-defined isometric cubic or octahedral crystals. On a fresh, unweathered surface, the mineral displays a distinctive reddish, orangish, or brownish hue. However, exposure to air and moisture quickly alters this surface, producing a characteristic green or blue-green tarnish or patina composed of copper(II) carbonate, a condition often referred to as verdigris. The mineral has a specific gravity of 8.9 and a Mohs hardness of 2.5 to 3, making it relatively soft and dense. The world’s largest known deposits of native copper are found in the Keweenaw native copper deposits of Upper Michigan, which were major sources of copper during the 19th and early 20th centuries. Prehistoric peoples, including Native Americans, mined this copper on a small scale at these and many other locations, and isotopic analysis of artifacts has revealed extensive copper trading routes across North America. The first commercial mines in the Keweenaw Peninsula, a region nicknamed the “Copper Country,” opened in the 1840s. Another significant deposit exists at Coro Coro, Bolivia. A notable geological specimen, the Ontonagon Boulder, now held by the Smithsonian Institution, records evidence of native copper being transported by glacial action and deposited along the Ontonagon River in Michigan.

Reader's Guide

The mines of the Keweenaw native copper deposits of Upper Michigan were major copper producers in the 19th and early 20th centuries, and are the largest deposits of native copper in the world. Native Americans mined copper on a small scale at this and many other locations, and evidence using isotopic analysis exists of copper trading routes throughout North America among native peoples. The first commercial mines in the Keweenaw Peninsula (nicknamed the 'Copper Country' and 'Copper Island') opened in the 1840s. Isle Royale in western Lake Superior was also a site of many tons of native copper. Some of it was extracted by native peoples, but only one of several commercial attempts at mining turned a profit there. A geological record of native copper being dragged by a glacier and deposited on the west branch of the Ontonagon River up river from Lake Superior is seen in the Ontonagon Boulder in the possession of the Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution. Another major native copper deposit is in Coro Coro, Bolivia.

Did You Know?

• Native copper is one of the few metallic elements to occur in native form.
• The largest deposits of native copper in the world are in the Keweenaw native copper deposits of Upper Michigan.
• Native Americans mined copper on a small scale at the Keweenaw and many other locations, with evidence of copper trading routes throughout North America.
• The Ontonagon Boulder, a record of native copper dragged by a glacier, is held by the Smithsonian Institution.