igneous
Basalt River Cobble
Extrusive Mafic Igneous Rock (primarily Plagioclase and Pyroxene)
Hardness: 6-7 on Mohs scale; Color: Dark grey to charcoal; Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to charcoal
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to charcoal; Luster: Dull/Earthy; 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 very near the Earth's surface. This specimen has been rounded by fluvial (river) erosion over thousands of years.
Uses & applications
Commonly used in construction aggregate, road base, and as decorative landscaping stones. Smooth river cobbles are also popular for hot stone massage therapy and decorative garden features.
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, Mars, and Venus.
Field identification & locations
Identify in the field by its dark color, heavy weight (relative to size), and lack of visible crystals. It is ubiquitous in volcanic regions and riverbeds downstream from volcanic mountain ranges.
