igneous
Basalt Pebble
Extrusive mafic igneous rock
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull or earthy when weathered; Structure: Fine-grained (aphanitic), sometimes vesicular; Cleavage: None/fractional; Density: 2.8–3.0 g/cm³.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Dull or earthy when weathered
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull or earthy when weathered; Structure: Fine-grained (aphanitic), sometimes vesicular; Cleavage: None/fractional; Density: 2.8–3.0 g/cm³.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava (mafic) at or very near the Earth's surface. This specimen has been rounded and smoothed by water erosion in a river or coastal environment.
Uses & applications
Commonly used in construction for road base, concrete aggregate, and railroad ballast. Smooth pebbles are popular for landscaping, aquarium decoration, and 'hot stone' massage therapy.
Geological facts
Basalt is the most common rock type in Earth's crust and makes up the majority of the ocean floor. It is also found extensively on the Moon and Mars.
Field identification & locations
Identify by its dark color, heavy weight for its size, and absence of visible large crystals. Commonly found on beaches or in stream beds near volcanic regions.
