igneous
Dark river pebble, likely basalt
Basalt, a fine-grained volcanic rock composed mainly of plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectBlack to dark gray, rounded and water-worn, with a dull to subtly waxy luster and fine crystalline texture; Mohs hardness about 5–6, generally massive with no visible cleavage, and specific gravity roughly 2.8–3.0.
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Physical properties
Black to dark gray, rounded and water-worn, with a dull to subtly waxy luster and fine crystalline texture; Mohs hardness about 5–6, generally massive with no visible cleavage, and specific gravity roughly 2.8–3.0.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes; its rounded shape indicates stream or beach transport.
Uses & applications
Used extensively as crushed aggregate, road stone, riprap, railway ballast, and landscaping material; attractive polished pieces may be used decoratively, but this specimen has little gem value.
Geological facts
Basalt is the most abundant volcanic rock in Earth’s oceanic crust and commonly forms during seafloor spreading and hotspot volcanism.
Field identification & locations
Look for a dense, dark, fine-grained rock that does not fizz in dilute acid and may attract a weak magnet if magnetite is present. Common in volcanic terrains and gravel deposits worldwide; identification is provisional from the photograph because grain size and mineral content are not fully visible.
