metamorphic
Banded gneiss
Quartz–feldspar–biotite gneiss (exact minerals uncertain from the photo)
AI-generated identification · may be incorrectCoarse, elongated specimen with alternating light and dark mineral bands, typical of gneissic foliation. Common components are quartz and feldspar with dark biotite or amphibole; hardness is roughly 6–7 Mohs, with a generally dull to glassy luster and no prominent cleavage as a whole rock.
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Physical properties
Coarse, elongated specimen with alternating light and dark mineral bands, typical of gneissic foliation. Common components are quartz and feldspar with dark biotite or amphibole; hardness is roughly 6–7 Mohs, with a generally dull to glassy luster and no prominent cleavage as a whole rock.
Formation & geological history
Forms when pre-existing rocks undergo high-grade heat and pressure, segregating minerals into bands. Gneisses range widely in age, including Precambrian examples; this specimen’s age and locality cannot be determined from the image.
Uses & applications
Used as decorative and dimension stone, aggregate, and a collectible field specimen.
Geological facts
The light-and-dark compositional banding is called gneissic banding and distinguishes many gneisses from more uniformly foliated schists.
Field identification & locations
Look for coarse, alternating light and dark bands; unlike sedimentary layers, these bands are metamorphic mineral segregation. Gneiss occurs in ancient continental shields and mountain belts worldwide.
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