igneous
Weathered brown pebble, likely basalt or andesite
Fine-grained mafic to intermediate volcanic rock; exact identification requires testing
AI-generated identification · may be incorrectTypically dark brown-gray, dull to slightly earthy luster, dense and fine-grained, with no visible cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.6–3.0. The pictured surface appears strongly weathered, obscuring original minerals and texture.
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Physical properties
Typically dark brown-gray, dull to slightly earthy luster, dense and fine-grained, with no visible cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.6–3.0. The pictured surface appears strongly weathered, obscuring original minerals and texture.
Formation & geological history
Likely formed from rapidly cooled lava, probably during the Cenozoic or older, then rounded by stream, beach, or glacial transport and chemically weathered. The image alone cannot establish age or precise composition.
Uses & applications
Common basalt and andesite are used as crushed aggregate, road stone, riprap, and occasionally building stone; ordinary pebbles have minimal commercial value.
Geological facts
Basalt commonly contains plagioclase and pyroxene, with possible olivine or magnetite; andesite is richer in silica and commonly contains plagioclase, amphibole, or pyroxene.
Field identification & locations
Look for a dense, fine-grained texture, dark color, lack of foliation, and possible tiny plagioclase specks; a streak, magnet test, hardness test, and thin section would improve identification. Common worldwide in volcanic regions and river gravels.
