igneous
Basalt
Mafic extrusive igneous rock
Hardness: 6 (Mohs); Color: Dark grey, greenish-black, or brown when weathered; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0
- Hardness
- 6 (Mohs)
- Color
- Dark grey, greenish-black, or brown when weathered
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs); Color: Dark grey, greenish-black, or brown when weathered; 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 surface. This specimen appears to be a weathered fluvial or glacial cobble.
Uses & applications
Commonly used as crushed stone for road base, railroad ballast, concrete aggregate, and high-quality paving stones. It can also be used in rock wool insulation.
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 by its heavy weight (relative to size), dark color, and fine-grained texture. It often shows a reddish-brown 'weathering rind' as the iron minerals oxidize.
