igneous
Basalt (River Stone/Pebble)
Basalt (Extrusive Igneous Rock)
Hardness: 6-7 (Mohs scale); Color: Dark grey to black, weathering to brown/green; Luster: Dull/Matte; Crystal structure: Aphanitic (fine-grained) to porphyritic with small visible phenocrysts; Cleavage: None/Irregular; Specific gravity: 2.8–3.0.
- Hardness
- 6-7 (Mohs scale)
- Color
- Dark grey to black, weathering to brown/green
- Luster
- Dull/Matte
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark grey to black, weathering to brown/green; Luster: Dull/Matte; Crystal structure: Aphanitic (fine-grained) to porphyritic with small visible phenocrysts; Cleavage: None/Irregular; Specific gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. This specimen has been rounded by fluvial (river) or coastal erosion processes. Geological age varies by locality but occurs throughout Earth's history.
Uses & applications
Used widely in construction as crushed stone (aggregate) for roads and railways. Smooth river versions are popular for decorative landscaping, 'hot stone' massage therapy, and aquarium decor.
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 and Mars. When cooled in large masses, it can form striking hexagonal columns like the Giant's Causeway.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and fine-grained texture. It will not react with acid (unlike limestone). Commonly found in volcanic regions, riverbeds, and beaches.
