metamorphic
Greenstone
Protolith of basalt, diabase, or gabbro; often contains minerals like chlorite, actinolite, and epidote
Hardness: 5-6 (Mohs); Color: Pale to dark green, sometimes with grayish-white flecks; Luster: Dull to earthy; Crystal structure: Usually fine-grained/aphanitic; Cleavage: Often lacks distinct cleavage due to its massive structure; Specific gravity: 2.7-3.2
- Hardness
- 5-6 (Mohs)
- Color
- Pale to dark green, sometimes with grayish-white flecks
- Luster
- Dull to earthy
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Physical properties
Hardness: 5-6 (Mohs); Color: Pale to dark green, sometimes with grayish-white flecks; Luster: Dull to earthy; Crystal structure: Usually fine-grained/aphanitic; Cleavage: Often lacks distinct cleavage due to its massive structure; Specific gravity: 2.7-3.2
Formation & geological history
Formed through the low-grade metamorphism of mafic igneous rocks (basalt or gabbro) under high pressure and low temperatures (greenschist facies). This process often occurs in subduction zones or mountain-building regions over millions of years.
Uses & applications
Used primarily as crushed stone for road construction, fill material, and railway ballast. Polished varieties are sometimes used for architectural stone or landscape decoration.
Geological facts
The green color comes from minerals like chlorite and epidote that replace original volcanic minerals. Many 'greenstone belts' around the world are billions of years old and are associated with major gold and nickel deposits.
Field identification & locations
Identify in the field by its distinct green hue, fine-grained texture, and toughness (difficult to break). It often occurs in ancient volcanic terrains. Collectors should look for unique mineral inclusions or variations in shading.
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