igneous
Rounded red granite cobble
Granite, typically quartz–alkali feldspar–plagioclase–biotite; approximate composition SiO₂-rich felsic intrusive rock
AI-generated identification · may be incorrectCoarse-grained, pink-red and dark-speckled, with a dull to slightly glassy luster; hardness about 6–7 Mohs, no preferred cleavage as a rock, and very high durability. The rounded shape reflects stream abrasion, not original crystal form.
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Physical properties
Coarse-grained, pink-red and dark-speckled, with a dull to slightly glassy luster; hardness about 6–7 Mohs, no preferred cleavage as a rock, and very high durability. The rounded shape reflects stream abrasion, not original crystal form.
Formation & geological history
Formed by slow crystallization of felsic magma deep in the continental crust, commonly Precambrian to Phanerozoic in age; later exposed and transported by glacial or fluvial processes. The surrounding large cobble appears to be a different, weathered greenish rock.
Uses & applications
Used as crushed aggregate, building stone, monuments, paving, countertops, and landscaping stone; this waterworn specimen is mainly of educational or decorative interest.
Geological facts
The red color commonly comes from iron-bearing potassium feldspar, while black flecks are usually biotite or amphibole. Individual crystals are visible because cooling was slow underground.
Field identification & locations
Identify it by visible interlocking quartz and feldspar grains, pink coloration, and black mica; it is nonvesicular and does not fizz in dilute acid. Common in continental shields and granitic mountain regions worldwide.
