igneous
Rounded dark gray cobble
Likely basalt or fine-grained mafic volcanic rock; exact identification requires testing
AI-generated identification · may be incorrectDense, fine-grained, dull to weakly satin luster, gray-brown exterior, and rounded water-worn shape; basalt hardness is about Mohs 6, with little or no visible cleavage. The smooth surface may reflect weathering or landscaping abrasion.
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Physical properties
Dense, fine-grained, dull to weakly satin luster, gray-brown exterior, and rounded water-worn shape; basalt hardness is about Mohs 6, with little or no visible cleavage. The smooth surface may reflect weathering or landscaping abrasion.
Formation & geological history
Likely formed from rapidly cooled mafic lava, commonly Cenozoic or older, then rounded by stream or glacial transport. It may also be a landscaping stone collected from a local gravel deposit.
Uses & applications
Used as landscaping gravel, riprap, paving aggregate, and construction stone; attractive pieces may be kept as common geological specimens. Typical value is about $1–$10, generally less than $20 unless locality-specific.
Geological facts
Basalt commonly contains plagioclase feldspar, pyroxene, and often olivine or magnetite. Its dark color and fine texture distinguish it from coarse-grained gabbro.
Field identification & locations
Look for a uniformly fine texture, high density, dark gray to black color, and possible tiny feldspar crystals; a streak, magnetism, acid test, and fresh broken surface would improve confidence. Common worldwide, especially volcanic regions and glacial or river gravels.
