igneous
Basalt
Basalt (extrusive volcanic rock)
Hardness: 6 on Mohs scale; Color: dark grey to black; Luster: dull to sub-metallic; Crystal structure: fine-grained (aphanitic); Cleavage: none; Specific gravity: 2.8–3.0.
- Hardness
- 6 on Mohs scale
- Color
- dark grey to black
- Luster
- dull to sub-metallic
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Physical properties
Hardness: 6 on Mohs scale; Color: dark grey to black; Luster: dull to sub-metallic; Crystal structure: fine-grained (aphanitic); Cleavage: none; Specific gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. This specimen appears water-worn or tumbled.
Uses & applications
Commonly used as crushed stone for road base, railroad ballast, and concrete aggregate. High-quality basalt is used in stone carvings and as thermal insulation (rock wool).
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 on the Moon and Mars.
Field identification & locations
Identified by its dark color, heavy weight for its size, and absence of visible large crystals. Common in volcanic regions and riverbeds downstream from volcanic sources.
