igneous
Basalt
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6-7 on Mohs scale; Color: Dark grey to black, often weathering to green or brown; Luster: Dull to sub-metallic; Crystal Structure: Fine-grained (aphanitic) or porphyritic; Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to black, often weathering to green or brown
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black, often weathering to green or brown; Luster: Dull to sub-metallic; Crystal Structure: Fine-grained (aphanitic) or porphyritic; 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 moon. Most basalt on Earth formed during the Cenozoic and Mesozoic eras, though it occurs throughout geological history.
Uses & applications
Extensively used in construction for aggregate, road base, railroad ballast, and concrete. It is also used as a dimension stone for floor tiles and monuments, or as high-quality stone wool for 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 remarkably common on other terrestrial bodies like the Moon, Mars, and Venus.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and fine-grained texture with occasional small visible crystals (phenocrysts). Common in volcanic regions such as Iceland, Hawaii, and the Deccan Traps. Collectors look for 'columnar basalt' or specimens containing vesicles filled with secondary minerals like zeolites.
