igneous
Dark gray river pebble, likely basalt or mudstone
Tentatively fine-grained basalt (plagioclase–pyroxene), though the image cannot exclude mudstone or weathered andesite
AI-generated identification · may be incorrectDark gray, fine-grained, dull to weakly subvitreous surface with pale polygonal weathering or fracture veins; likely hardness about 5–6 for basalt and no visible cleavage. Specific gravity would typically be about 2.8–3.1.
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Physical properties
Dark gray, fine-grained, dull to weakly subvitreous surface with pale polygonal weathering or fracture veins; likely hardness about 5–6 for basalt and no visible cleavage. Specific gravity would typically be about 2.8–3.1.
Formation & geological history
If basalt, it formed from rapidly cooled mafic lava, commonly in volcanic flows, probably Cenozoic or older; rounded edges indicate later stream or shoreline abrasion. The pale network may represent mineral-filled cracks or surface weathering.
Uses & applications
Common basalt is used as crushed aggregate, road stone, riprap, and occasionally decorative stone; an ordinary pebble has little commercial value.
Geological facts
Basalt commonly contains microscopic plagioclase and pyroxene crystals that are not resolvable in this photograph. A vinegar test is generally negative, helping distinguish it from carbonate rock.
Field identification & locations
Look for a uniformly fine texture, high density, dark color, and absence of obvious layering; a hand lens, streak, hardness, and specific-gravity tests would improve identification. Dark rounded pebbles occur widely in volcanic terrains and river gravels.
