igneous
Weathered granite or granitic gneiss
Granite, chiefly quartz–feldspar–mica; possibly weakly gneissic
AI-generated identification · may be incorrectMedium- to coarse-grained, gray with pale feldspar/quartz and dark biotite or amphibole; vitreous quartz and blocky feldspar grains, no obvious cleavage in the whole rock. Typical hardness is about 6–7 Mohs and specific gravity 2.6–2.8; the rounded shape reflects weathering, not crystal form.
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Physical properties
Medium- to coarse-grained, gray with pale feldspar/quartz and dark biotite or amphibole; vitreous quartz and blocky feldspar grains, no obvious cleavage in the whole rock. Typical hardness is about 6–7 Mohs and specific gravity 2.6–2.8; the rounded shape reflects weathering, not crystal form.
Formation & geological history
An intrusive felsic rock that crystallized slowly from magma deep in the crust, commonly hundreds of millions to billions of years ago, then was exposed and rounded by erosion and chemical weathering.
Uses & applications
Used widely as crushed aggregate, building stone, monuments, countertops, and landscaping material; this specimen has little commercial value unless unusually attractive or proven to be a rare gneiss or pegmatite.
Geological facts
The image shows a weathered, compositionally mottled cobble rather than a diagnostic crystal or gemstone. Dark patches may be biotite-rich minerals or weathering products.
Field identification & locations
Identify it by visible interlocking light quartz/feldspar and dark mica/amphibole grains; a streak test is usually ineffective, while a fresh broken surface and hand lens are useful. Common worldwide in continental crust and glacial or stream gravels.
