metamorphic
Greenstone (Metabaltic/Greenschist)
Chlorite schist / Meta-basalt
Hardness (Mohs): 3.5 - 4.5. Color: Dark green, grayish-green, mottled with earthy brown oxidation. Luster: Dull to sub-vitreous. Crystal structure: Microcrystalline to foliated. Cleavage: Poorly developed schistosity. Specific Gravity: 2.8 - 3.0.
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness (Mohs): 3.5 - 4.5. Color: Dark green, grayish-green, mottled with earthy brown oxidation. Luster: Dull to sub-vitreous. Crystal structure: Microcrystalline to foliated. Cleavage: Poorly developed schistosity. Specific Gravity: 2.8 - 3.0.
Formation & geological history
Formed from the low-grade metamorphism of mafic igneous rocks (such as basalt or gabbro) under relatively low temperatures and pressures. This creates greenish minerals like chlorite, epidote, and actinolite.
Uses & applications
Historically used for stone tools (axes) due to its toughness. Today it is used as crushed stone for road construction, riprap for erosion control, and occasionally as architectural building stone.
Geological facts
Ancient civilizations highly valued certain varieties of greenstone for making 'celt' axes. The green color is primarily due to the presence of the mineral chlorite.
Field identification & locations
Identify by its characteristic dull 'sage' or 'forest' green color and fine-grained texture. It will not react with acid and is generally harder than slate but softer than quartzite. Found commonly in ancient mountain belts and shield areas.
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