igneous
Dark rounded pebble, likely basalt
Basalt (fine-grained mafic volcanic rock; commonly plagioclase, pyroxene, and olivine)
AI-generated identification · may be incorrectDark gray-green, smooth and rounded from abrasion, with a dull to faintly subvitreous luster and tiny pale mineral grains. Typical basalt hardness is about Mohs 6, with no obvious cleavage and specific gravity around 2.8–3.0.
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Physical properties
Dark gray-green, smooth and rounded from abrasion, with a dull to faintly subvitreous luster and tiny pale mineral grains. Typical basalt hardness is about Mohs 6, with no obvious cleavage and specific gravity around 2.8–3.0.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic fields and oceanic crust. Its exact age cannot be determined from the photograph; the pebble may have been transported and naturally polished by water or ice.
Uses & applications
Used as crushed aggregate, roadstone, riprap, paving stone, and occasionally decorative landscaping material. This ordinary pebble has little commercial or collector value.
Geological facts
The specimen’s rounded shape indicates mechanical weathering and transport, while its dark color suggests mafic composition. Similar-looking rocks can include greenstone, diabase, or dark metamorphic rocks.
Field identification & locations
Look for a dense, fine-grained texture, dark color, lack of visible large crystals, and weak magnetism in some specimens. Common in volcanic regions, river gravels, beaches, and glacial deposits.
