igneous
Basalt
Basalt (Aphanitic mafic volcanic rock)
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic; Structure: Fine-grained/Aphanitic (crystals too small to see with naked eye); Cleavage: None; Specific Gravity: 2.8 - 3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic; Structure: Fine-grained/Aphanitic (crystals too small to see with naked eye); 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. It can be any age from Hadean to modern volcanic eruptions.
Uses & applications
Major use in construction as aggregate (crushed stone) for roads, railroad ballast, and concrete. Also used for high-quality floor tiles, 'hot stone' massage therapy, and as a raw material for stone wool insulation.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found extensively on the Moon (the lunar maria) and Mars.
Field identification & locations
Identify in the field by its dark color, heavy weight (relative to size), and lack of visible crystals. It is found globally in volcanic regions like Iceland, Hawaii, and the Pacific Northwest. Collectors look for vesicles (gas bubbles) filled with secondary minerals like quartz or zeolites.
