igneous
Basalt (Greenstone/Trap Rock)
Basalt (dominated by plagioclase feldspar and pyroxene)
Hardness: 6 on Mohs scale; Color: Dark green to blackish-green; Luster: Dull to waxy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0.
- Hardness
- 6 on Mohs scale
- Color
- Dark green to blackish-green
- Luster
- Dull to waxy
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Physical properties
Hardness: 6 on Mohs scale; Color: Dark green to blackish-green; Luster: Dull to waxy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of low-viscosity magnesium-rich and iron-rich lava. Green-tinted specimens often result from low-grade metamorphism (chloritization) of original basaltic lava flows, commonly from the Precambrian or Mesozoic eras.
Uses & applications
Used primarily in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Exceptionally smooth or distinct green pieces are used as aquarium stones or for lapidary tumbling.
Geological facts
Basalt is the most common rock type in Earth's crust and makes up most of the ocean floor. It is also found extensively on the Moon and Mars.
Field identification & locations
Identify by its dark, uniform fine-grained texture and heavy weight for its size. It often exhibits a greenish hue if it has undergone hydrothermal alteration. Commonly found in volcanic regions and on beaches near ancient lava flows.
