igneous
Rounded dark pebble, likely basalt or fine-grained volcanic rock
Basalt (probable; composition dominated by plagioclase and pyroxene, with possible olivine)
AI-generated identification · may be incorrectDark gray-brown, fine-grained, dull to weakly satin luster, massive and nonfoliated; typically Mohs 6–7, with no visible cleavage and specific gravity about 2.8–3.0. Its elongated rounded form reflects water abrasion rather than crystal shape.
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Physical properties
Dark gray-brown, fine-grained, dull to weakly satin luster, massive and nonfoliated; typically Mohs 6–7, with no visible cleavage and specific gravity about 2.8–3.0. Its elongated rounded form reflects water abrasion rather than crystal shape.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood basalts. The specimen is likely geologically ancient bedrock fragment reshaped in a modern stream or beach environment.
Uses & applications
Common basalt is used as crushed aggregate, road stone, riprap, concrete material, and occasionally dimension stone; this small pebble has mainly decorative or educational value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Dark color and fine grain distinguish it from lighter, quartz-rich rocks such as granite.
Field identification & locations
Look for a heavy, dark, uniformly fine-grained pebble that commonly attracts a weak magnet if magnetite is present; a streak test and fresh broken surface help distinguish it from shale or slag. Basalt pebbles occur widely in volcanic terrains, rivers, beaches, and glacial deposits.
