igneous
Dark river-worn pebble, likely basalt
Basalt (fine-grained mafic volcanic rock; chiefly plagioclase and pyroxene, with possible olivine and magnetite)
AI-generated identification · may be incorrectVery dark gray to black, matte to faintly subvitreous, fine-grained, and smoothly water-rounded; hardness about 6 on Mohs scale, with no visible cleavage and typically high density (about 2.8–3.0 g/cm³). The image cannot confirm composition, and it could also be another dark volcanic rock.
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Physical properties
Very dark gray to black, matte to faintly subvitreous, fine-grained, and smoothly water-rounded; hardness about 6 on Mohs scale, with no visible cleavage and typically high density (about 2.8–3.0 g/cm³). The image cannot confirm composition, and it could also be another dark volcanic rock.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and flood-basalt provinces. The pebble’s rounded shape indicates later transport and abrasion by water or waves; its exact age is indeterminate.
Uses & applications
Crushed basalt is widely used for aggregate, road metal, railroad ballast, concrete, and rock wool; attractive pieces may be used as landscaping or low-value curios. This ordinary pebble has little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. It is commonly weakly magnetic when magnetite-rich.
Field identification & locations
Look for a uniform dark color, fine texture, heavy feel, and lack of visible quartz or large crystals; a streak test is usually gray, and it should not fizz with dilute acid. Common in volcanic regions and streambeds worldwide.
