igneous
Basalt River Pebble
Basalt (Aphanitic Mafic Igneous Rock)
Hardness: 6-7 (Mohs); Color: Black to dark gray; Luster: Dull or waxy when wet; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0
- Hardness
- 6-7 (Mohs)
- Color
- Black to dark gray
- Luster
- Dull or waxy when wet
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Physical properties
Hardness: 6-7 (Mohs); Color: Black to dark gray; Luster: Dull or waxy when wet; 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 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 as crushed stone for road bases and concrete aggregate; smaller pebbles are used for landscaping, decorative aquariums, and hot stone massage therapy.
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 on the Moon and Mars.
Field identification & locations
Identify by its dark color, heavy weight (relative to size), and fine-grained texture. Found abundantly in volcanic regions or riverbeds downstream from volcanic sources.
