Metamorphic
Greenstone
Metabasalt / Greenschist facies rock
Hardness: 5-7 (Mohs); Color: dull green to dark gray-green; Luster: matte to waxy; Texture: fine-grained, sometimes foliated or porphyritic with darker phenocrysts; Composition: chlorite, epidote, actinolite.
- Hardness
- 5-7 (Mohs)
- Color
- dull green to dark gray-green
- Luster
- matte to waxy
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Physical properties
Hardness: 5-7 (Mohs); Color: dull green to dark gray-green; Luster: matte to waxy; Texture: fine-grained, sometimes foliated or porphyritic with darker phenocrysts; Composition: chlorite, epidote, actinolite.
Formation & geological history
Formed through low-grade metamorphism of mafic igneous rocks like basalt. Subjected to moderate pressure and temperature in subduction zones or mountain-building environments, often during the Proterozoic or Archean eras.
Uses & applications
Used historically for stone tools and weapons (celt axes); currently used for road ballast, decorative garden stones, and occasionally carved ornaments or jewelry.
Geological facts
Many greenstones were once ancient seafloor basalts that have been recycled by tectonic plates. Large 'greenstone belts' are famous for containing major gold and silver deposits.
Field identification & locations
Identify by its characteristic greenish hue (due to chlorite) and hardness. It is typically harder than shale but lacks the distinct layering of many other metamorphic rocks. Often found as rounded beach or river cobbles.
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