igneous
Basalt Cobble
Basalt
Hardness: 6 Mohs; Color: Dark grey to black, weathering to brown; Luster: Dull to earthy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0
- Hardness
- 6 Mohs
- Color
- Dark grey to black, weathering to brown
- Luster
- Dull to earthy
Identified More igneous →
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Physical properties
Hardness: 6 Mohs; Color: Dark grey to black, weathering to brown; 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 appears to be a water-worn river cobble.
Uses & applications
Crushed basalt is used in construction for aggregate, road base, and railroad ballast. In polished form, it is used for floor tiles and monuments.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floors. It is also found on the Moon, Mars, and Venus.
Field identification & locations
Look for a heavy, dark-colored, fine-grained rock. River-worn specimens like this one are identifying by their rounded, smooth surfaces caused by water erosion. Common in volcanic regions and riverbeds down-stream from basaltic flows.
