igneous
Basalt
Basalt
Hardness: 6 (Mohs scale); Color: Black to dark grey; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific gravity: 2.8–3.0; often contains small phenocrysts of olivine or pyroxene.
- Hardness
- 6 (Mohs scale)
- Color
- Black to dark grey
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Black to dark grey; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific gravity: 2.8–3.0; often contains small phenocrysts of olivine or pyroxene.
Formation & geological history
Formed from the rapid cooling of low-viscosity magnesium-rich and iron-rich lava (mafic) at or very near the Earth's surface. Common in oceanic crust, volcanic islands, and flood basalt provinces.
Uses & applications
Extensively used in construction for road base, concrete aggregate, railroad ballast, and dimension stone for architectural accents. Also used as a raw material for rock wool insulation.
Geological facts
Basalt is the most common rock on Earth's surface and makes up most of the ocean floor. It is also found on the Moon, Mars, and Venus. The famous Giant's Causeway in Northern Ireland is made of basaltic columns.
Field identification & locations
Identified in the field by its dark color, heavy weight (high density), and fine-grained texture. Found near volcanic regions or areas with historical lava flows. Collectors look for vesicles (gas bubbles) or embedded semi-precious minerals like peridot.
