igneous
Basalt
Mafic volcanic rock (primarily plagioclase feldspar and pyroxene)
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull/earthy; Crystal structure: Aphanitic (fine-grained/microscopic crystals); Cleavage: None (conchoidal fracture); Specific gravity: 2.8–3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Dull/earthy
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull/earthy; Crystal structure: Aphanitic (fine-grained/microscopic crystals); Cleavage: None (conchoidal fracture); Specific gravity: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or nearby the surface of a terrestrial planet or a moon. Most common rock type in the Earth's crust (oceanic crust).
Uses & applications
Extensively used in construction for aggregate, road base, railroad ballast, and high-quality concrete. Also used for acid-resistant laboratory equipment and as a base for stone wool insulation.
Geological facts
Basalt is the most common rock type on the surfaces of Mercury, Venus, the Moon, and Mars. Giant's Causeway in Northern Ireland is a famous example of columnar basalt jointing.
Field identification & locations
Identify by its dark color, heavy weight (relative to size), and fine-grained texture with no visible crystals to the naked eye. Commonly found in volcanic regions and ocean floor outcrops. Collectors look for vesicles or 'amygdules' filled with secondary minerals like zeolites.
