igneous
Basalt
Basalt
Hardness: 6 (Mohs scale); Color: Dark gray to black, often weathering to brownish or green; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained), sometimes vesicular; Cleavage: None; Specific gravity: 2.8–3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark gray to black, often weathering to brownish or green
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark gray to black, often weathering to brownish or green; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained), sometimes vesicular; 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 is the most common volcanic rock on Earth, covering much of the ocean floor and large continental provinces.
Uses & applications
Extensively used in construction for aggregate, road base, concrete, and railroad ballast. It is also used as dimension stone for tiles and pavers, and in the production of stone wool insulation.
Geological facts
Basalt is found on the Moon, Mars, and Venus. On Earth, the Giants Causeway in Northern Ireland is a famous example of columnar basalt jointing caused by the cooling of thick lava flows.
Field identification & locations
Identify by its dark color, heavy weight (density), and lack of visible crystals. It is ubiquitous in volcanic regions and oceanic crust. Collectors usually find it on beaches or near volcanic outcrops.
