igneous
Dark weathered river pebble, likely basalt
Basalt; fine-grained mafic rock dominated by plagioclase and pyroxene, commonly with minor olivine and iron oxides
AI-generated identification · may be incorrectDark gray-brown, dull to weakly vitreous, rounded and water-worn with a fine-grained texture; typical hardness is about 5–6 Mohs, with no visible cleavage and specific gravity around 2.8–3.0. The image is insufficient to confirm basalt versus another dark volcanic rock.
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Physical properties
Dark gray-brown, dull to weakly vitreous, rounded and water-worn with a fine-grained texture; typical hardness is about 5–6 Mohs, with no visible cleavage and specific gravity around 2.8–3.0. The image is insufficient to confirm basalt versus another dark volcanic rock.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic flows and oceanic crust; most examples range from recent to Precambrian in age. Rounding indicates transport in a stream, beach, or glacial environment.
Uses & applications
Used extensively as crushed aggregate, road material, riprap, railroad ballast, and dimension stone; ordinary pebbles have little jewelry or collector value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and commonly forms oceanic crust and columnar-jointed lava flows. Its dark color reflects abundant ferromagnesian minerals and iron oxides.
Field identification & locations
Check for a uniformly fine-grained texture, high density, dark color, and possible tiny pale plagioclase crystals; a streak, magnetism test, and fresh broken surface would improve identification. Common worldwide, especially volcanic regions and river gravels.
