igneous
Basalt River Stone
Basalt (Mafic volcanic rock, primarily Plagioclase and Pyroxene)
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal black with white mineral weathering or streaks; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0.
- Hardness
- 6 (Mohs scale)
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal black with white mineral weathering or streaks; Luster: Dull to sub-metallic; 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 specific specimen has been further shaped by fluvial erosion (river action), smoothing its edges into a rounded 'cobble' or 'pebble.'
Uses & applications
Commonly used as high-density construction aggregate, in the production of rock wool insulation, for landscaping, and as 'hot stones' for massage therapy due to high heat retention.
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. When basalt cools slowly, it can form spectacular hexagonal columns like those at the Giant's Causeway.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and fine-grained texture. It often shows 'desert varnish' or white surface oxidation when weathered. Found globally in volcanic regions and riverbeds downstream from volcanic sources.
