igneous
Basalt (Beach Pebble)
Extrusive mafic igneous rock
Hardness: 5-6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-vitreous; Structure: Aphanitic (fine-grained); Cleavage: None; Specific Gravity: 2.8–3.0
- Hardness
- 5-6 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-vitreous; Structure: Aphanitic (fine-grained); Cleavage: None; Specific Gravity: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at the Earth's surface. This specific specimen has been rounded and smoothed by water erosion (wave action) in a river or coastal environment.
Uses & applications
Commonly used as decorative landscaping stones, aquarium gravel, mass-produced river rock, and in construction as crushed aggregate for road base or concrete.
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 extensively on the Moon and Mars.
Field identification & locations
Identify by its heavy weight relative to size, dark matte color, and lack of visible crystal grains. Found globally on beaches and in volcanic regions.
