igneous
Mottled granite or granitic gneiss pebble
Granite; chiefly quartz, feldspar, and biotite/hornblende
AI-generated identification · may be incorrectMedium- to coarse-grained, speckled cream, gray, black, and brown rock with vitreous quartz, pale feldspar, and dark mica or amphibole. Hardness is approximately 6–7 Mohs; no prominent cleavage as a whole, though feldspar and mica show cleavage; density is about 2.6–2.8 g/cm³.
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Physical properties
Medium- to coarse-grained, speckled cream, gray, black, and brown rock with vitreous quartz, pale feldspar, and dark mica or amphibole. Hardness is approximately 6–7 Mohs; no prominent cleavage as a whole, though feldspar and mica show cleavage; density is about 2.6–2.8 g/cm³.
Formation & geological history
Formed by slow crystallization of silica-rich magma deep in the crust, generally during Precambrian to Phanerozoic geologic time. Its smooth, rounded shape indicates transport and abrasion in a river, beach, or glacial environment.
Uses & applications
Granite is widely used for building stone, countertops, monuments, paving, and aggregate; attractive polished pieces may be used as decorative or educational specimens.
Geological facts
The black grains are likely biotite mica and/or hornblende, while the light grains are feldspar and quartz. A strongly layered or foliated texture would instead indicate granitic gneiss, a metamorphic equivalent.
Field identification & locations
Identify it by its interlocking visible crystals, hardness, and lack of bubbling in dilute acid; a steel knife generally does not scratch it. Commonly found in granite terrains, stream gravels, beaches, and glacial deposits.
