igneous
Basalt River Cobble
Basalt
Hardness: 5-6 (Mohs); Color: Dark grey to black; Luster: Dull to earthy; Crystal structure: Aphanitic (fine-grained); Specific Gravity: 2.8-3.0
- Hardness
- 5-6 (Mohs)
- Color
- Dark grey to black
- Luster
- Dull to earthy
Identified More igneous →
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark grey to black; Luster: Dull to earthy; Crystal structure: Aphanitic (fine-grained); 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 rounded by fluvial (river) or glacial erosion over thousands of years.
Uses & applications
Used primarily as crushed stone for road base, railroad ballast, and concrete aggregate. Also common in landscaping and decorative river rock beds.
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 in the field by its dark color, fine-grained texture (no visible crystals), and heavy weight for its size. Often found in riverbeds, glacial tills, or near volcanic fields.
