igneous
Rounded gray river cobble, likely basaltic-andesite or andesite
Fine-grained intermediate to mafic volcanic rock; commonly plagioclase ± pyroxene, with possible alteration minerals
AI-generated identification · may be incorrectHardness about 5.5–6.5 Mohs; medium gray, dull to weakly granular luster, massive and nonfoliated, with no obvious cleavage. The smooth rounded shape indicates prolonged water or sediment abrasion.
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Physical properties
Hardness about 5.5–6.5 Mohs; medium gray, dull to weakly granular luster, massive and nonfoliated, with no obvious cleavage. The smooth rounded shape indicates prolonged water or sediment abrasion.
Formation & geological history
Formed when lava cooled at or near Earth’s surface, likely during a relatively young volcanic episode; exact age cannot be determined from the photograph. It was later transported and rounded in a stream, beach, or glacial environment.
Uses & applications
Commonly used as landscaping stone, riprap, aggregate, and construction fill; generally has little jewelry value unless unusually patterned or polished.
Geological facts
The specimen appears dense and fine-grained, with scattered lighter mineral grains and possible weathering spots. A fresh broken surface would better distinguish basalt from andesite.
Field identification & locations
Common in volcanic terrains and river gravels worldwide, including the Pacific Northwest, Iceland, Japan, and the Andes. Field tests include checking for high density, lack of visible quartz, and a streakless surface; a magnet may show weak attraction if magnetite is present.
