igneous
Dark river pebble, likely basalt
Basalt; fine-grained mafic volcanic rock, typically plagioclase + pyroxene ± olivine
AI-generated identification · may be incorrectHardness about 6–7 Mohs; dark gray to black, dull to subtly granular, with no visible crystals or cleavage. Dense, fine-grained, and commonly magnetic if iron-rich.
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Physical properties
Hardness about 6–7 Mohs; dark gray to black, dull to subtly granular, with no visible crystals or cleavage. Dense, fine-grained, and commonly magnetic if iron-rich.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, often in oceanic crust, lava flows, or volcanic islands. Its exact age cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, roadstone, riprap, paving, and occasionally landscaping stone; generally not used as a gemstone.
Geological facts
The rounded, smooth shape indicates prolonged abrasion by moving water or sediment. The uniformly fine texture is consistent with basalt, though dark andesite or another volcanic rock is also possible.
Field identification & locations
Common in volcanic regions and river or beach gravels worldwide; identify by its dark color, high density, fine texture, and lack of layering. A streak, magnet test, and fresh broken surface would improve identification.
