metamorphic
Quartz–feldspar–mica metamorphic rock (likely gneiss)
Granitic gneiss; chiefly quartz (SiO₂), feldspar, and biotite/muscovite mica
AI-generated identification · may be incorrectThe specimen is coarse-grained, white to gray, with dark mica-rich patches and apparent foliation/banding. Quartz hardness is 7, feldspar 6–6.5, and mica 2–3; overall luster is mostly vitreous to pearly, with no obvious cleavage across the whole rock.
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Physical properties
The specimen is coarse-grained, white to gray, with dark mica-rich patches and apparent foliation/banding. Quartz hardness is 7, feldspar 6–6.5, and mica 2–3; overall luster is mostly vitreous to pearly, with no obvious cleavage across the whole rock.
Formation & geological history
Formed when granitic or sedimentary precursor rock underwent intense heat and pressure, producing recrystallization and compositional banding. Such rocks commonly formed during Precambrian to Paleozoic mountain-building events, though age cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, dimension stone, and occasionally architectural facing; attractive polished pieces may be collected, but this example has little gemstone value.
Geological facts
Gneiss is recognized by alternating light quartz–feldspar layers and darker mica or amphibole layers, unlike massive granite. It may grade into migmatite where partial melting occurred.
Field identification & locations
Common in continental shields and deeply eroded mountain belts, including the Canadian Shield, Scandinavia, India, and the Appalachians. Look for foliation, interlocking coarse grains, and visible mica flakes; a fresh surface and hardness testing improve identification.
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