metamorphic
Granitic gneiss or weathered granite cobble
Quartz–feldspar–mica rock; likely gneiss (compositionally granite-derived)
AI-generated identification · may be incorrectCoarse-grained, mottled gray-white rock with translucent quartz, pale feldspar, and dark biotite or amphibole; minerals show weak foliation/banding, with no obvious cleavage as a whole. Individual minerals are about Mohs 6–7; luster is mostly vitreous to pearly, and specific gravity is roughly 2.6–2.8.
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Physical properties
Coarse-grained, mottled gray-white rock with translucent quartz, pale feldspar, and dark biotite or amphibole; minerals show weak foliation/banding, with no obvious cleavage as a whole. Individual minerals are about Mohs 6–7; luster is mostly vitreous to pearly, and specific gravity is roughly 2.6–2.8.
Formation & geological history
Likely formed from an ancient granitic protolith that underwent high-temperature metamorphism, producing compositional segregation and foliation; its exact geologic age cannot be determined from the image. The rounded shape indicates prolonged transport in a river, beach, or glacial deposit.
Uses & applications
Used as durable aggregate, riprap, landscaping stone, and occasionally dimension stone; attractive pieces may be polished for decorative use. This ordinary cobble has little commercial gemstone value.
Geological facts
The light bands are quartz and feldspar-rich domains, while dark patches commonly contain biotite or hornblende. Weathering has altered feldspar and concentrated surface stains and dark mineral grains.
Field identification & locations
Identify it by its interlocking coarse grains, quartz-rich glassy areas, abundant feldspar, and possible faint banding; a streak test is unhelpful, while a fresh broken surface and hand lens are useful. Common in continental mountain belts and glacial or fluvial gravels.
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