igneous
Basalt
Mafic Igneous Rock (primarily Plagioclase Feldspar and Pyroxene)
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black; 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. Common in oceanic crust and volcanic regions.
Uses & applications
Used widely in construction as crushed stone for road bases, railroad ballast, and concrete aggregate. Select varieties are used in 'stonewashed' finishings.
Geological facts
Basalt is the most common rock type in the Earth's crust and also makes up most of the surface of the Moon (the dark 'seas' or maria) and Mars.
Field identification & locations
Identify by its heavy weight (relative to size), dark color, and lack of visible crystals. Commonly found in volcanic fields or near coastal cliffs in tectonic hotspots.
