igneous
Rounded dark cobble in gravel
Likely mafic volcanic rock, such as basalt or diabase; exact identification is uncertain from the image
AI-generated identification · may be incorrectDark gray-green, fine-grained surface with scattered lighter flecks; likely Mohs hardness 5–6, with a dull to weakly lustrous appearance. The rounded form suggests abrasion; crystal structure and cleavage are not distinguishable in the photograph.
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Physical properties
Dark gray-green, fine-grained surface with scattered lighter flecks; likely Mohs hardness 5–6, with a dull to weakly lustrous appearance. The rounded form suggests abrasion; crystal structure and cleavage are not distinguishable in the photograph.
Formation & geological history
If basalt or diabase, it formed when iron- and magnesium-rich magma cooled; the cobble was later rounded by transport in water or glacial ice. Its age cannot be determined from the image.
Uses & applications
Ordinary cobbles are used in landscaping, drainage, and aggregate; this specimen has little commercial value unless its identity or locality makes it collectible.
Geological facts
Dark color and fine texture are consistent with a mafic rock, but the visible speckling could also reflect a different rock type or surface coating.
Field identification & locations
Compare a fresh broken surface: basalt is typically fine-grained, while diabase has more visible interlocking crystals. Dark rounded cobbles occur in river gravels, beaches, and glacial deposits worldwide.
