igneous
Rounded gray stream cobble, likely weathered basalt
Basalt (fine-grained mafic volcanic rock; identification uncertain from photograph)
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, dull to weakly subvitreous, with hardness about 5–6, no visible cleavage, and specific gravity around 2.8–3.0. The specimen appears rounded and strongly weathered, with a pale gray oxidation rind or surface coating.
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Physical properties
Typically dark gray to black, fine-grained, dull to weakly subvitreous, with hardness about 5–6, no visible cleavage, and specific gravity around 2.8–3.0. The specimen appears rounded and strongly weathered, with a pale gray oxidation rind or surface coating.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its exact geological age cannot be determined from the image; stream rounding indicates later erosion and transport.
Uses & applications
Common basalt is used as crushed aggregate, road stone, riprap, paving, and occasionally building stone; it has little jewelry value unless it contains attractive vesicles, zeolite, or agate fillings.
Geological facts
Basalt is the most abundant volcanic rock and commonly contains plagioclase feldspar, pyroxene, and sometimes olivine. Weathering can produce brown, orange, or gray iron-rich coatings.
Field identification & locations
Look for a heavy, fine-grained dark rock with tiny crystals and no obvious layering; a streak, fresh broken surface, and magnet test can help distinguish it from other stream rocks. Common in volcanic regions and basalt-bearing river gravels.
