igneous
Rounded granitic cobble (granite)
Granite; chiefly quartz, alkali feldspar, plagioclase, and biotite/hornblende
AI-generated identification · may be incorrectHardness about 6–7 Mohs; gray, fine-to-medium granular texture, with dull to slightly vitreous luster. Massive, nonfoliated, and lacking cleavage as a whole; density typically 2.6–2.8 g/cm³.
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Physical properties
Hardness about 6–7 Mohs; gray, fine-to-medium granular texture, with dull to slightly vitreous luster. Massive, nonfoliated, and lacking cleavage as a whole; density typically 2.6–2.8 g/cm³.
Formation & geological history
An intrusive igneous rock crystallized slowly from silica-rich magma, commonly during Precambrian to Phanerozoic continental crust formation. Its smooth oval shape indicates prolonged river, beach, or glacial abrasion.
Uses & applications
Used widely as building stone, aggregate, curbing, monuments, countertops, and landscaping material; attractive polished pieces may be collected but are generally low value.
Geological facts
The visible speckling reflects interlocking crystals rather than sedimentary grains. Granite commonly contains radioactive potassium-40 in feldspar, though ordinary specimens are not hazardous.
Field identification & locations
Identify it by its interlocking gray crystals, hardness, and inability to fizz with dilute acid; a hand lens may reveal glassy quartz and pale feldspar. Common worldwide, especially in river gravels and glacial deposits.
