igneous
Rounded granite or granitic gneiss cobble
Granite–gneiss, chiefly quartz, feldspar, and mica
AI-generated identification · may be incorrectCoarse, pale gray-pink granular texture with dark mineral flecks and faint banding; likely Mohs 6–7, vitreous to dull luster, interlocking crystals, and no obvious cleavage as a whole rock. The smooth, oval shape reflects stream or glacial abrasion.
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Physical properties
Coarse, pale gray-pink granular texture with dark mineral flecks and faint banding; likely Mohs 6–7, vitreous to dull luster, interlocking crystals, and no obvious cleavage as a whole rock. The smooth, oval shape reflects stream or glacial abrasion.
Formation & geological history
Likely an ancient continental igneous rock, possibly later metamorphosed into gneiss; mineral grains formed during Precambrian-to-younger crustal cooling and recrystallization. Its present rounded form developed in a river, beach, or glacial environment.
Uses & applications
Commonly used as aggregate, road stone, landscaping rock, riprap, and building dimension stone; attractive pieces may be polished for decorative use. This ordinary cobble has little commercial value.
Geological facts
The speckled appearance comes mainly from quartz and feldspar with scattered biotite or hornblende. Faint streaks may indicate gneissic foliation rather than sedimentary layering.
Field identification & locations
Identify it by visible interlocking grains, abundant pale feldspar/quartz, dark mica or amphibole, and hardness sufficient to scratch glass. Common in streambeds and glacial deposits worldwide; a streak, acid, or thin-section test would refine identification.
