igneous
Basalt (River Rock Cobble)
Extrusive mafic volcanic rock (Basalt)
Hardness: 5-6 (Mohs scale), Color: Dark grey to charcoal, Luster: Dull/Earthy, Crystal Structure: Aphanitic (fine-grained), Cleavage: None/Irregular, Specific Gravity: 2.8–3.0
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Physical properties
Hardness: 5-6 (Mohs scale), Color: Dark grey to charcoal, Luster: Dull/Earthy, Crystal Structure: Aphanitic (fine-grained), 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 very near the Earth's surface. This specific specimen has been Rounded by fluvial (river) erosion over many years.
Uses & applications
Used in construction as aggregate, for road base, in concrete, and as decorative river stone in landscaping. High-quality specimens are sometimes used for hot stone massage therapy.
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 by its heavy weight (high density), fine-grained texture, and dark color. This water-worn cobble is typical of riverbeds and glacial deposits. Found globally in volcanic regions.
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