igneous
Basalt
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained crystals generally not visible to the naked eye); Specific gravity: 2.8-3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black, weathering to brown
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained crystals generally not visible to the naked eye); 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. Basalts occur in all geological ages from the Archean to the Holocene.
Uses & applications
Used extensively in construction as road aggregate, railroad ballast, and in concrete. It is also used as dimension stone for tiles and facades, and occasionally in jewelry as ornmental stone.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. Large-scale areas of basalt are known as flood basalts, such as the Deccan Traps or Columbia River Basalt Group.
Field identification & locations
Identify by its heavy weight (density), dark color, and fine-grained texture. Found globally in volcanic regions. Look for small holes called vesicles which may contain secondary minerals.
