igneous
Basalt pebble
Fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectDark gray to black, rounded and water-worn, with a dull to subtly vitreous luster and fine-grained texture; typical hardness is about 5–6 Mohs, with no visible cleavage or crystals at this scale.
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Physical properties
Dark gray to black, rounded and water-worn, with a dull to subtly vitreous luster and fine-grained texture; typical hardness is about 5–6 Mohs, with no visible cleavage or crystals at this scale.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its exact age cannot be determined from the photograph.
Uses & applications
Common basalt is used as crushed aggregate, road base, riprap, building stone, and occasionally polished as a decorative material; this small pebble has negligible commercial value.
Geological facts
The specimen’s smooth, compact, non-layered appearance and uniformly dark fine grain are consistent with basalt, though fine-grained andesite or another dark volcanic rock is possible.
Field identification & locations
Identify it by its black-gray color, heavy feel, fine texture, and lack of obvious grains; a streak test, density measurement, and thin-section or chemical analysis would confirm it. Basalt pebbles occur in volcanic regions, river gravels, beaches, and glacial deposits worldwide.
