igneous
Copper-bearing Basalt with Epidote and Quartz
Native copper in basalt amygdaloidal matrix
Hardness varies by mineral (Basalt: 6, Quartz: 7, Copper: 3); Colors include dark purplish-brown (basalt), white/grey (quartz), yellowish-green (epidote), and reddish-orange/metallic (native copper and cuprite); Luster ranges from vitreous in quartz to metallic in copper and dull in the basalt matrix;…
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Physical properties
Hardness varies by mineral (Basalt: 6, Quartz: 7, Copper: 3); Colors include dark purplish-brown (basalt), white/grey (quartz), yellowish-green (epidote), and reddish-orange/metallic (native copper and cuprite); Luster ranges from vitreous in quartz to metallic in copper and dull in the basalt matrix; Cryptocrystalline to massive crystal structure.
Formation & geological history
Formed in ancient volcanic environments where basaltic lava flows developed gas cavities (vesicles) later filled with hydrothermal fluids depositing quartz, epidote, and native copper. Typically Precambrian to Keweenawan age (approx. 1.1 billion years old).
Uses & applications
Specimen collecting, lapidary purposes, and historical ore extraction for copper.
Geological facts
Stones like this are typical of the Keweenaw Peninsula in Michigan, where glacial action tumbled and rounded chunks of copper-bearing amygdaloidal basalt, often referred to as 'float copper' or 'greenstone-copper mixes'.
Field identification & locations
Easily identified by the dark volcanic host rock contrasted with bright green epidote patches and metallic copper veins or dendritic flecks. Common in glacial drift around the Great Lakes region of North America.
