igneous
Basalt Pebble
Extrusive igneous rock (Mafic)
Hardness: 6 on the Mohs scale. Color: Dark gray to black. Luster: Dull to sub-vitreous. Crystal structure: Aphanitic (fine-grained), often containing tiny plagioclase feldspar and pyroxene crystals. Cleavage: None. Specific gravity: 2.8 - 3.0.
- Hardness
- 6 on the Mohs scale
- Color
- Dark gray to black
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6 on the Mohs scale. Color: Dark gray to black. Luster: Dull to sub-vitreous. Crystal structure: Aphanitic (fine-grained), often containing tiny plagioclase feldspar and pyroxene crystals. Cleavage: None. Specific gravity: 2.8 - 3.0.
Formation & geological history
Formed from the rapid cooling of low-viscosity, magnesium- and iron-rich lava exposed at or very near the Earth's surface, often shaped into a smooth pebble by water action in a riverbed or shoreline.
Uses & applications
Used in construction as crushed stone for road bases, concrete aggregate, railroad ballast, and occasionally as decorative landscaping stones.
Geological facts
Basalt is the most common rock type in the Earth's crust underlying the world's ocean basins, and it is also extensively found on the Moon (forming the lunar maria) and Mars.
Field identification & locations
Easily identified by its dark, fine-grained appearance and high density. Found worldwide in volcanic regions and water-worn sedimentary deposits where volcanic rocks have eroded.
