igneous
Rounded quartz-rich cobble, likely granite-derived stone
Granite or granitic gneiss, composed mainly of quartz, feldspar, and minor mica
AI-generated identification · may be incorrectHardness about 6–7 Mohs; pale gray-green to tan with rusty iron staining, dull to weakly granular luster, and no obvious cleavage on the worn surface. Its smooth, rounded shape reflects water or glacial abrasion rather than crystal form.
Identified More igneous →
Explore Rounded quartz-rich cobble, likely granite-derived stone in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness about 6–7 Mohs; pale gray-green to tan with rusty iron staining, dull to weakly granular luster, and no obvious cleavage on the worn surface. Its smooth, rounded shape reflects water or glacial abrasion rather than crystal form.
Formation & geological history
Originally crystallized as intrusive felsic magma, commonly during Precambrian to Paleozoic continental crust formation; later weathering and transport rounded it into a cobble. Exact rock type cannot be confirmed without a fresh broken surface or microscopy.
Uses & applications
Comparable material is used as aggregate, landscaping stone, riprap, and decorative cobbles; this individual specimen has modest display value.
Geological facts
The reddish-brown patches are likely oxidized iron minerals along weathered grains or fractures. The surface appears strongly weathered, obscuring the original mineral texture.
Field identification & locations
Look for visible interlocking quartz and feldspar grains, with scattered dark mica, on a fresh surface; granite fizzes weakly or not at all with acid. Common in streambeds, glacial deposits, and eroded uplands worldwide.
