igneous
Rounded volcanic cobble, possibly basalt
Basaltic volcanic rock; exact identification is uncertain from the photograph
AI-generated identification · may be incorrectDark gray, fine-grained and weathered, with pale surface coatings or vesicle fillings; likely Mohs hardness 5–6, dull to faintly waxy luster, massive texture, and no obvious cleavage. The rounded shape indicates water or wave abrasion.
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Physical properties
Dark gray, fine-grained and weathered, with pale surface coatings or vesicle fillings; likely Mohs hardness 5–6, dull to faintly waxy luster, massive texture, and no obvious cleavage. The rounded shape indicates water or wave abrasion.
Formation & geological history
Likely formed as an extrusive lava flow when basaltic magma cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older; its precise age and source cannot be determined visually. It was later transported and rounded in a stream, beach, or glacial deposit.
Uses & applications
Common basalt is used for aggregate, road base, riprap, paving, and landscaping; this individual piece has little commercial value unless it proves to be an unusual mineral-bearing specimen.
Geological facts
The white patches may be weathering, quartz, calcite, zeolite, or lichen rather than the rock’s primary composition. A streak, fresh broken surface, acid test, magnetism, and thin-section analysis would improve identification.
Field identification & locations
Look for a dark, dense, fine-grained rock, commonly weakly magnetic, with tiny crystals visible under magnification. Basaltic cobbles are widespread in volcanic regions, river gravels, beaches, and glacial deposits.
