igneous
Rounded basalt cobble
Basalt (fine-grained mafic volcanic rock)
AI-generated identification · may be incorrectTypically 5–6 Mohs; dark gray to brown, dull to slightly waxy, fine-grained crystalline texture, with vesicles and weathering rinds. Likely contains plagioclase, pyroxene, olivine, and iron oxides; no obvious cleavage as a whole rock.
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Physical properties
Typically 5–6 Mohs; dark gray to brown, dull to slightly waxy, fine-grained crystalline texture, with vesicles and weathering rinds. Likely contains plagioclase, pyroxene, olivine, and iron oxides; no obvious cleavage as a whole rock.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, generally in the Cenozoic or older volcanic terrains; its smooth shape indicates prolonged river, beach, or glacial abrasion.
Uses & applications
Used as aggregate, riprap, paving stone, and occasionally as a decorative or lapidary material; ordinary cobbles have little commercial value.
Geological facts
The cavities are vesicles, gas bubbles preserved during lava cooling, while the pale line may be a mineral vein or weathering seam.
Field identification & locations
Identify by its dense dark fine-grained body, rounded abrasion, scattered vesicles, and lack of obvious large crystals; common in volcanic regions and downstream gravel deposits.
