igneous
Gray Basalt
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 5-6 (Mohs scale); Color: Dark gray to black; Luster: Dull/Earthy; Crystal Structure: Aphanitic (fine-grained); Cleavage: None; Specific Gravity: 2.8-3.0.
- Hardness
- 5-6 (Mohs scale)
- Color
- Dark gray to black
- Luster
- Dull/Earthy
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Dark gray to black; Luster: Dull/Earthy; Crystal Structure: Aphanitic (fine-grained); 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. This specimen appears to be of the vesicular or fine-grained variety commonly found in volcanic plateaus.
Uses & applications
Commonly used in construction as crushed stone for road base, concrete aggregate, railroad ballast, and sometimes in high-quality floor tiles or as 'hot stones' for therapies.
Geological facts
Basalt is the most common rock type on Earth's crust and makes up most of the ocean floor. It is also found on the Moon, Mars, and Venus.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and fine-grained texture. It often breaks with a conchoidal fracture. Found in volcanic regions like the Pacific Northwest, Hawaii, or Iceland.
