metamorphic
Greenstone
Metabasalt / Greenschist facies rock
Hardness: 5-6 (Mohs); Color: dull green to deep olive; Luster: earthy to slightly waxy; Crystal structure: fine-grained (aphanitic) to schistose; Specific Gravity: 2.8–3.0.
- Hardness
- 5-6 (Mohs)
- Color
- dull green to deep olive
- Luster
- earthy to slightly waxy
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Physical properties
Hardness: 5-6 (Mohs); Color: dull green to deep olive; Luster: earthy to slightly waxy; Crystal structure: fine-grained (aphanitic) to schistose; Specific Gravity: 2.8–3.0.
Formation & geological history
Formed through the low-grade metamorphism of mafic igneous rocks (such as basalt or gabbro) at low temperature and high pressure. This process typically occurs in ancient oceanic crust subduction zones or mountain-building environments.
Uses & applications
Historically used for making stone tools (axes/celts); currently used for crushed stone in construction, decorative landscaping, and occasionally as a base for architectural carvings.
Geological facts
The green color comes from minerals like chlorite, epidote, and actinolite formed during the metamorphosis. Many 'Greenstone Belts' are billions of years old and are associated with major gold and nickel deposits.
Field identification & locations
Identify by its distinctive dull green hue and lack of visible individual mineral grains to the naked eye. Commonly found in eroded ancient mountain chains such as the Appalachians or the Canadian Shield.
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