igneous
Rounded dark river pebble, likely basalt
Basalt (fine-grained mafic volcanic rock; identification tentative from appearance)
AI-generated identification · may be incorrectHardness about 6–6.5 Mohs; dark olive-gray to brown, dull to weakly silky luster, very fine-grained interlocking crystals, and no obvious cleavage. Likely composed mainly of plagioclase feldspar and pyroxene, with possible iron oxides or minor olivine.
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Physical properties
Hardness about 6–6.5 Mohs; dark olive-gray to brown, dull to weakly silky luster, very fine-grained interlocking crystals, and no obvious cleavage. Likely composed mainly of plagioclase feldspar and pyroxene, with possible iron oxides or minor olivine.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. The smooth, rounded shape indicates prolonged abrasion in a river, beach, or glacial environment; its age cannot be determined from the photograph.
Uses & applications
Common basalt is used as crushed aggregate, roadstone, riprap, railroad ballast, and occasionally dimension stone. This pebble has minimal commercial value but may be useful as a teaching or landscape specimen.
Geological facts
Its dark color and fine texture distinguish it from coarse-grained gabbro; weathering can produce the brown or olive surface seen here.
Field identification & locations
Look for a dense, fine-grained texture, high specific gravity, weak magnetism, and tiny pale feldspar grains; a streak, hardness, and acid test would improve identification. Basalt pebbles occur worldwide, especially in volcanic or mafic-rock drainage basins.
