igneous
Basalt (Beach Pebble)
Extrusive Mafic Igneous Rock
Hardness: 6 (Mohs scale); Color: Black to dark gray; Luster: Dull to sub-metallic (sparkles due to phenocrysts); Crystal Structure: Aphanitic (fine-grained) with possible micro-phenocrysts; Specific Gravity: 2.8 - 3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Black to dark gray
- Luster
- Dull to sub-metallic (sparkles due to phenocrysts)
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Physical properties
Hardness: 6 (Mohs scale); Color: Black to dark gray; Luster: Dull to sub-metallic (sparkles due to phenocrysts); Crystal Structure: Aphanitic (fine-grained) with possible micro-phenocrysts; Specific Gravity: 2.8 - 3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at the Earth's surface. This specific specimen has been rounded and polished by coastal wave action (abrasion) over many years.
Uses & applications
Crushed basalt is used in construction for road base and railroad ballast; in decorative gardens as 'river rock'; and occasionally in hot stone massage therapy.
Geological facts
Basalt is the most common rock type on 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 by its dark color, heavy weight (density), and fine-grained texture. On beaches, it often looks jet black when wet. Look for it in volcanic coastal regions like the Pacific Northwest, Hawaii, or Iceland.
