Cuprite is a reddish- to almost black-colored, copper oxide mineral (Cu2O). It most frequently occurs as microcrystalline coatings on pre-existing copper metal or Cu-bearing sulfides. It sometimes occurs as nice, bright red, lustrous, transparent crystals ("ruby copper ore"). Cuprite may be mistaken for hematite or cinnabar, but cuprite is always associated with copper minerals. It occurs in the supergene zone, the near-surface portion of mineral deposits where oxidizing conditions occur.
As native copper weathers, a coating of cuprite forms, followed later by a black coating (tenorite). After that, cuprite and tenorite can combine with water and atmospheric carbon dioxide to form green coatings of malachite.
Cuprite (reddish) on native copper (4.2 cm across) from the Portage Lake Volcanic Series of northern Michigan, USA.
Cuprite (blackish) on native copper from the Poteryaevskoe Mine, northwestern end of the Altay Mountains, southwestern Siberia, Russia, north-central Asia. The Poteryaevskoe Mine exploited the Rubtsovsk Cu-Zn-Pb Deposit, a subaqueous, volcanogenic massive sulfide deposit in the Kamenevsk Group, upper Frasnian Stage, ~mid-Upper Devonian. (public display, Hillman Hall of Minerals and Gems, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA) (Some info. provided by Kudryavtseva et al., 1979, Geologiya i Razvedka 1979(10): 82-88.)