igneous
Basalt River Cobble
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6 (Mohs scale); Color: Gray to dark gray; Luster: Dull or earthy; Crystal structure: Aphanitic (fine-grained); Cleavage: None/Conchoidal fracture; Specific gravity: 2.8 - 3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Gray to dark gray
- Luster
- Dull or earthy
Identified More igneous →
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Physical properties
Hardness: 6 (Mohs scale); Color: Gray to dark gray; Luster: Dull or earthy; Crystal structure: Aphanitic (fine-grained); Cleavage: None/Conchoidal fracture; 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 specific specimen has been smoothed and rounded by water erosion in a river or coastal environment.
Uses & applications
Used extensively in construction as aggregate, in road base, for decorative landscaping stones (river rocks), and crushed for railroad ballast. High-quality basalt is also used for cast-basalt liners.
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 on the Moon, Mars, and Venus.
Field identification & locations
Identify by its characteristic heavy weight for its size, dark gray color, and fine-grained texture lacking visible crystals. Commonly found in volcanic regions and stream beds.
