igneous
Rounded dark pebble, possibly weathered basalt
Fine-grained mafic volcanic rock; likely basalt, but identification is uncertain from the photograph
AI-generated identification · may be incorrectBrown-gray, dull to weakly earthy luster, subrounded and apparently fine-grained with no visible crystals or cleavage. Estimated hardness about 5–6 Mohs and specific gravity roughly 2.8–3.1 if basaltic.
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Physical properties
Brown-gray, dull to weakly earthy luster, subrounded and apparently fine-grained with no visible crystals or cleavage. Estimated hardness about 5–6 Mohs and specific gravity roughly 2.8–3.1 if basaltic.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic flows; its age cannot be determined visually. The rounded shape indicates transport and abrasion by water or sediment.
Uses & applications
Common basalt is used as aggregate, roadstone, riprap, paving, and ballast; attractive polished pieces may be collected, but this specimen appears low commercial grade.
Geological facts
Its brown surface may reflect weathering and oxidation of iron-bearing minerals. Similar-looking pebbles can include andesite, dark sandstone, or altered volcanic rock.
Field identification & locations
Look for a dense, non-layered texture, faintly crystalline surface, and lack of strong acid reaction; a streak, magnet test, fresh break, and close inspection can help distinguish basalt from sedimentary rocks. Basalt pebbles are widespread in volcanic and river environments.
