igneous
Basalt
Aphanitic Mafic Igneous Rock
Hardness: 6 (Mohs); Color: Gray to dark charcoal; Luster: Dull/Earthy; Structure: Fine-grained (Aphanitic); Specific Gravity: 2.8-3.0; Cleavage: None.
- Hardness
- 6 (Mohs)
- Color
- Gray to dark charcoal
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6 (Mohs); Color: Gray to dark charcoal; Luster: Dull/Earthy; Structure: Fine-grained (Aphanitic); Specific Gravity: 2.8-3.0; Cleavage: None.
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 shows signs of fluvial erosion, giving it a rounded cobble shape.
Uses & applications
Commonly used as crushed stone for road base, concrete aggregate, railroad ballast, and high-tensile mineral wool 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 found in abundance on the Moon and Mars.
Field identification & locations
Identify in the field by its heavy weight (density) compared to other rocks, dark color, and lack of visible large crystals. Commonly found in volcanic fields or riverbeds near volcanic activity.
