igneous
River-worn Basalt
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6 (Mohs scale); Color: dark grey to black; Luster: dull to sub-glassy; Crystal structure: fine-grained (aphanitic) reflecting rapid cooling; Specific gravity: approx. 2.9-3.1.
- Hardness
- 6 (Mohs scale)
- Color
- dark grey to black
- Luster
- dull to sub-glassy
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Physical properties
Hardness: 6 (Mohs scale); Color: dark grey to black; Luster: dull to sub-glassy; Crystal structure: fine-grained (aphanitic) reflecting rapid cooling; Specific gravity: approx. 2.9-3.1.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the surface of a terrestrial planet or the moon. The rounded shape indicates long-term mechanical weathering in a river or fluvial environment.
Uses & applications
Used extensively in construction for aggregate, road base, and railroad ballast. Exceptionally smooth specimens are used in stone massage (basalt stones) or decorative landscaping.
Geological facts
Basalt is the most common rock type in the Earth's crust, making up most of the ocean floors. It is also found on the Moon and Mars.
Field identification & locations
Identify in the field by its dark color, heavy weight compared to similar-sized rocks, and fine-grained texture. Commonly found in volcanic regions or riverbeds downstream from volcanic sources.
