metamorphic
Greenstone (Altered Mafic Rock)
Greenschist / Meta-basalt
Hardness: 5-6 (Mohs scale); Color: Dull forest green to grey-green; Luster: Dull or waxy; Crystal structure: Fine-grained, microcrystalline; Cleavage: None to poor; Specific gravity: 2.8-3.0
- Hardness
- 5-6 (Mohs scale)
- Color
- Dull forest green to grey-green
- Luster
- Dull or waxy
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Dull forest green to grey-green; Luster: Dull or waxy; Crystal structure: Fine-grained, microcrystalline; Cleavage: None to poor; Specific gravity: 2.8-3.0
Formation & geological history
Formed through low-grade metamorphism of mafic igneous rocks (such as basalt) under low temperature and moderate pressure. Often associated with ancient ocean crust subduction zones.
Uses & applications
Used historically for stone tools (axes and adzes) due to its toughness; used today as a decorative building stone, crushed stone for construction, and as aquarium or landscaping decor.
Geological facts
The green color is primarily due to the presence of chlorite, actinolite, and epidote minerals. It is a major component of ancient continental shields and greenstone belts which host significant gold deposits.
Field identification & locations
Identify by its distinctive dull green hue and fine-grained texture. Commonly found in riverbeds or coastal areas near metamorphic belts, particularly in the Pacific Northwest and Appalachian regions of the US.
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