metamorphic
Native Copper in Basalt
Amygdaloidal Basalt with Native Copper (Cu)
Hardness: 2.5-3 (copper) / 5-6 (basalt), Color: Reddish-brown metallic inclusions in dark grey/black matrix, Luster: Metallic for inclusions; dull for matrix, Cleavage: None, Specific Gravity: High (approx 8.9 for copper)
- Hardness
- 2
- Luster
- Metallic for inclusions
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Physical properties
Hardness: 2.5-3 (copper) / 5-6 (basalt), Color: Reddish-brown metallic inclusions in dark grey/black matrix, Luster: Metallic for inclusions; dull for matrix, Cleavage: None, Specific Gravity: High (approx 8.9 for copper)
Formation & geological history
Formed through hydrothermal precipitation where copper-rich fluids filled gas bubbles (vesicles) in volcanic basalt. Most common in the Keweenaw Peninsula, typically 1.1 billion years old (Mesoproterozoic).
Uses & applications
Historical source of industrial copper, lapidary use for jewelry (cabochons), and highly prized by mineral collectors.
Geological facts
This type of specimen is often referred to as a 'Lake Superior Agate' associate or 'Copper Pebble'. The Keweenaw Peninsula is the only place on Earth where large-scale native copper was found in this specific basaltic formation.
Field identification & locations
Identify by the high weight relative to size (high density) and the distinct metallic reddish-green copper patches against a dark stone. Commonly found on beaches of Lake Superior.
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