igneous
Rounded granitic cobble
Granite to granodiorite; quartz–feldspar–mica aggregate
AI-generated identification · may be incorrectCoarse crystalline, pale gray-tan with rusty weathering, speckled quartz/feldspar and dark mica; typically Mohs 6–7, vitreous to dull luster, no cleavage as a rock, and specific gravity about 2.6–2.8.
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Physical properties
Coarse crystalline, pale gray-tan with rusty weathering, speckled quartz/feldspar and dark mica; typically Mohs 6–7, vitreous to dull luster, no cleavage as a rock, and specific gravity about 2.6–2.8.
Formation & geological history
A plutonic igneous rock crystallized slowly from silica-rich magma deep in the crust, commonly during Precambrian to younger continental episodes; later exposed and rounded by erosion, transport, or glacial action.
Uses & applications
Used as crushed aggregate, road stone, building stone, countertops, monuments, and landscaping material; this ordinary cobble has little jewelry value.
Geological facts
The rounded shape indicates prolonged abrasion or transport. Rust-colored patches are likely oxidation of iron-bearing biotite or weathering minerals rather than metallic staining.
Field identification & locations
Identify it by visible interlocking quartz and feldspar grains, scattered dark mica, and a hard, nonporous texture; granite typically scratches glass and does not fizz with dilute hydrochloric acid. Common worldwide in continental mountain belts and glacial deposits.
