igneous
Basalt (Polished River Stone)
Basalt (Extrusive Mafic Volcanic Rock)
Hardness: 6-7 on Mohs scale; Color: Dark grey to black; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to black
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black; Luster: Dull to sub-vitreous; 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. This specific specimen has been further shaped by fluvial (river) erosion or artificial tumbling, giving it a smooth, rounded surface.
Uses & applications
Commonly used in construction as aggregate, in the manufacture of stone wool insulation, and as aesthetic decorative stones in landscaping or massage therapy (hot stones).
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 extensively on the Moon and Mars.
Field identification & locations
Identify in the field by its dark color, heavy weight (high density), and fine-grained texture. Large outcrops often show columnar jointing. It is frequently found as rounded 'cobbles' in riverbeds or on beaches near volcanic coastlines.
