igneous
Basalt River Cobble
Basalt (Aphanitic Mafic Volcanic Rock)
Hardness: 6-7 (Mohs scale); Color: Dark grey to charcoal, weathering to olive-brown; Luster: Dull/Earthy; Structure: Fine-grained (aphanitic) with a smooth, water-worn texture; Cleavage: None; Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 (Mohs scale)
- Color
- Dark grey to charcoal, weathering to olive-brown
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark grey to charcoal, weathering to olive-brown; Luster: Dull/Earthy; Structure: Fine-grained (aphanitic) with a smooth, water-worn texture; Cleavage: None; Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of low-viscosity magnesium and iron-rich lava. This specific specimen has been significantly rounded and smoothed by fluvial (river) erosion over thousands of years.
Uses & applications
Commonly used in landscaping as decorative 'river rock,' as crushed stone for road construction base, and in the production of high-strength insulation (mineral wool).
Geological facts
Basalt is the most common rock type in the Earth's crust and covers most 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 smooth, rounded shape if found in riverbeds. It is extremely common in the Pacific Northwest and volcanic regions. Collectors look for pieces with interesting mineral inclusions or vesicular 'gas bubble' holes.
