igneous
Basalt River Cobble
Basalt (Aphanitic Mafic Volcanic Rock)
Hardness: 6 (Mohs scale). Color: Dark grey to charcoal or greenish-black. Luster: Dull/Matte. Structure: Fine-grained (aphanitic) with a smooth tide-worn or river-worn texture. Cleavage: None. Specific Gravity: 2.8–3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to charcoal or greenish-black
- Luster
- Dull/Matte
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Physical properties
Hardness: 6 (Mohs scale). Color: Dark grey to charcoal or greenish-black. Luster: Dull/Matte. Structure: Fine-grained (aphanitic) with a smooth tide-worn or river-worn texture. Cleavage: None. 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 into a cobble by water erosion (alluvial process), likely within the last few thousand years, though the rock itself could be millions of years old.
Uses & applications
Common basalt is used in construction as aggregate for concrete and asphalt, railroad ballast, and landscaping. Smooth river stones are popular in 'hot stone' massage therapy and decorative garden masonry.
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 basalt cools slowly in thick layers, it can form unique hexagonal columns like those at the Giant's Causeway.
Field identification & locations
Identify in the field by its dark, uniform color, heavy weight relative to size, and lack of visible large crystals. It is commonly found in volcanic regions, riverbeds, and coastal beaches. This specimen shows a characteristic 'tumble' finish from natural water movement.
