igneous
Basalt cobble
Basalt; fine-grained mafic volcanic rock, typically plagioclase + pyroxene ± olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to slightly lusterous, with vesicles (gas bubbles) along the underside; hardness about 5–6 Mohs, massive with no visible cleavage, and typically dense (specific gravity ~2.8–3.0).
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Physical properties
Dark gray to black, fine-grained, dull to slightly lusterous, with vesicles (gas bubbles) along the underside; hardness about 5–6 Mohs, massive with no visible cleavage, and typically dense (specific gravity ~2.8–3.0).
Formation & geological history
Formed when iron- and magnesium-rich lava rapidly cooled at or near Earth’s surface, likely in a lava flow; its exact geological age cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road base, riprap, railway ballast, and building stone; attractive polished pieces are collected but usually have modest value.
Geological facts
The rounded shape and vesicular texture suggest water-worn basalt, possibly from a volcanic shoreline or river. Orange-brown areas may be iron-oxide weathering or alteration.
Field identification & locations
Identify it by its black fine-grained body, high density, lack of layering, and small gas cavities; common worldwide in volcanic regions, oceanic crust, Hawaii, Iceland, and the Pacific Northwest.
