igneous
Dark river-worn cobble, possibly basalt or fine-grained volcanic rock
Likely basaltic rock; exact identification requires a fresh surface and tests
AI-generated identification · may be incorrectDark gray to black, rounded and smoothly abraded, with dull to weakly glossy surfaces and pale weathering or mineral-rich patches. Basalt typically has Mohs hardness 5–6, no visible cleavage, very fine crystals, and specific gravity about 2.8–3.0.
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Physical properties
Dark gray to black, rounded and smoothly abraded, with dull to weakly glossy surfaces and pale weathering or mineral-rich patches. Basalt typically has Mohs hardness 5–6, no visible cleavage, very fine crystals, and specific gravity about 2.8–3.0.
Formation & geological history
If basalt, it formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic activity. Its rounded shape indicates prolonged transport and abrasion in a river, beach, or glacial setting.
Uses & applications
Common basalt is used as crushed aggregate, road stone, riprap, and occasionally building stone; this individual specimen is mainly suitable for collecting or display. It has little commercial value unless unusual texture, provenance, or mineralization is present.
Geological facts
The image shows a genuine geological cobble, but its exact composition cannot be confirmed visually because the surface is weathered and polished by abrasion. Pale patches could represent quartz, feldspar, carbonate, or weathering rind.
Field identification & locations
Look for a dense, fine-grained, dark interior; basalt may show tiny feldspar crystals or vesicles and commonly attracts a weak magnet. A streak, hardness, magnetism, density, and fresh fracture would distinguish it from andesite, gabbro, slate, or industrial slag.
