igneous
Rounded quartz-rich cobble, likely weathered granite or gneiss
Granite-derived granitoid; chiefly quartz, feldspar, and minor mica/amphibole
AI-generated identification · may be incorrectTan-gray, fine-to-medium granular texture with dark mineral patches and a dull-to-slightly sparkly luster; likely Mohs 6–7, with no obvious cleavage as a whole and irregular fracture. The rounded shape indicates prolonged water or sediment abrasion; exact identification is limited by the photograph.
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Physical properties
Tan-gray, fine-to-medium granular texture with dark mineral patches and a dull-to-slightly sparkly luster; likely Mohs 6–7, with no obvious cleavage as a whole and irregular fracture. The rounded shape indicates prolonged water or sediment abrasion; exact identification is limited by the photograph.
Formation & geological history
Likely eroded from an igneous or metamorphic outcrop and rounded in a river, beach, or gravel deposit. Its minerals commonly crystallized slowly from magma in Precambrian to younger continental crust, though the specimen’s age cannot be determined visually.
Uses & applications
Primarily a natural curiosity, teaching specimen, or decorative stone; unsuitable for gemstone use unless confirmed to have attractive mineral patterns. Similar granite is widely used as construction aggregate, paving, and dimension stone.
Geological facts
Quartz and feldspar grains commonly produce the mottled pale appearance, while dark biotite or amphibole creates the black-brown patches. Weathering can produce the tan iron-stained surface.
Field identification & locations
Look for interlocking visible grains, hardness sufficient to scratch glass, and quartz grains that do not fizz with dilute acid; strong fizz would instead suggest limestone. Common in river gravels and glacial deposits worldwide.
