igneous
Fine-grained volcanic pebble, likely basalt
Basalt; commonly composed of plagioclase, pyroxene, and minor olivine or magnetite
AI-generated identification · may be incorrectDark gray, fine-grained, rounded and naturally weathered, with small pale mineral grains and a dull to weakly glassy luster. Typical hardness is about 6 on the Mohs scale; massive basalt generally lacks visible cleavage and has a specific gravity near 2.8–3.0.
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Physical properties
Dark gray, fine-grained, rounded and naturally weathered, with small pale mineral grains and a dull to weakly glassy luster. Typical hardness is about 6 on the Mohs scale; massive basalt generally lacks visible cleavage and has a specific gravity near 2.8–3.0.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic activity; the rounded shape indicates transport by water or glacial action. The photograph cannot establish a precise age or distinguish basalt from closely related andesite with certainty.
Uses & applications
Used as crushed aggregate, road stone, riprap, paving, and dimension stone; attractive polished pieces are used decoratively. Common pebbles generally have negligible commercial value.
Geological facts
The fine texture reflects rapid cooling, while pale specks may be plagioclase feldspar or weathering products. Vesicles are absent or inconspicuous, suggesting relatively gas-poor lava.
Field identification & locations
Common in volcanic terrains and stream or glacial gravels worldwide, including Iceland, Hawaii, India, Scotland, and the Pacific Northwest. A dark, dense rock with tiny interlocking crystals and no layering supports basalt identification; a streak, magnet test, and fresh broken surface would help confirm it.