igneous
Rounded dark river pebble, possibly basalt
Basalt, a fine-grained mafic volcanic rock; exact identification is uncertain from the photograph
AI-generated identification · may be incorrectDark brown-black, smooth and rounded, with a dull to faintly subvitreous luster and tiny pale mineral flecks or vesicles. Basalt typically has Mohs hardness 5–6, no visible cleavage, fine interlocking plagioclase and pyroxene crystals, and specific gravity about 2.8–3.0.
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Physical properties
Dark brown-black, smooth and rounded, with a dull to faintly subvitreous luster and tiny pale mineral flecks or vesicles. Basalt typically has Mohs hardness 5–6, no visible cleavage, fine interlocking plagioclase and pyroxene crystals, and specific gravity about 2.8–3.0.
Formation & geological history
Likely formed when mafic lava cooled rapidly at or near Earth’s surface, then became rounded by stream or beach abrasion. Basalt occurs from Archean to modern ages, but this specimen’s age cannot be determined visually.
Uses & applications
Used as crushed aggregate, road stone, riprap, dimension stone, and occasionally as a decorative or lapidary material. An ordinary pebble generally has little commercial value.
Geological facts
The rounded shape indicates transport after formation; surface weathering may account for the brown coating. Similar-looking pebbles can also be andesite, gabbro, slag, or a weathered iron-rich rock.
Field identification & locations
Look for a dense, fine-grained texture, dark color, and occasional small vesicles; a streak, magnet test, and acid test can help distinguish alternatives, though laboratory analysis is needed for certainty. Common in volcanic regions and river gravels worldwide.
