metamorphic
Granitic gneiss
Banded quartz–feldspar–biotite gneiss; compositionally similar to granite
AI-generated identification · may be incorrectTypically Mohs 6–7, coarse-grained, with pale quartz and feldspar plus dark biotite/hornblende; vitreous quartz and pearly feldspar luster, weak-to-strong foliation, and no obvious cleavage as a whole rock.
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Physical properties
Typically Mohs 6–7, coarse-grained, with pale quartz and feldspar plus dark biotite/hornblende; vitreous quartz and pearly feldspar luster, weak-to-strong foliation, and no obvious cleavage as a whole rock.
Formation & geological history
Formed when granitic or sedimentary protoliths underwent high-grade regional metamorphism, recrystallizing and segregating minerals into streaks; commonly Precambrian to Paleozoic in age.
Uses & applications
Used as dimension stone, countertops, cladding, paving, and aggregate; attractive polished pieces are collected but generally have modest specimen value.
Geological facts
The mottled black-and-cream texture reflects mineral banding or stretched grains rather than crystal layering. It can resemble coarse granite, but aligned minerals and compositional streaking indicate metamorphism.
Field identification & locations
Common in ancient crystalline shields and mountain belts, including Scandinavia, Canada, the northeastern United States, India, and Brazil; inspect for foliation, alternating light/dark bands, and quartz-rich grains with a hand lens.
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