igneous
Basalt
Basalt (Mafic volcanic rock)
Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull (aphanitic texture); Crystal structure: Fine-grained/microcrystalline; Cleavage: None; Specific gravity: 2.8 - 3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark gray to black
- Luster
- Dull (aphanitic texture)
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull (aphanitic texture); Crystal structure: Fine-grained/microcrystalline; Cleavage: None; Specific gravity: 2.8 - 3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. It comprises more than 90% of all volcanic rock on Earth and characterizes oceanic crust.
Uses & applications
Extensively used in construction as aggregate (crushed stone) for road base, concrete, and railroad ballast. Also used as dimension stone for tiles and monuments, or for industrial 'stone wool' insulation.
Geological facts
Basalt is the most common rock in the solar system, making up the vast majority of the lunar maria on the Moon and the surface of Mars and Venus.
Field identification & locations
Identify in the field by its high density, dark color, and lack of visible large crystals. It is ubiquitous in volcanic regions like Iceland, Hawaii, and the Pacific Northwest. Collectors look for vesicles (air bubbles) containing zeolites or olivine crystals.
