metamorphic
Banded gneiss or gneissic schist
Quartz–feldspar–mica gneiss; exact mineral proportions require testing
AI-generated identification · may be incorrectDistinct light and dark compositional bands, fine-to-medium grain, dull to slightly silky luster, and strong foliation. Typical hardness is about 6–7 Mohs; quartz and feldspar have poor cleavage, while mica shows perfect basal cleavage.
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Physical properties
Distinct light and dark compositional bands, fine-to-medium grain, dull to slightly silky luster, and strong foliation. Typical hardness is about 6–7 Mohs; quartz and feldspar have poor cleavage, while mica shows perfect basal cleavage.
Formation & geological history
Formed when preexisting granite or sedimentary rock underwent high-grade regional metamorphism, causing minerals to recrystallize and segregate into bands. The specimen’s precise age cannot be determined visually; gneisses commonly range from Precambrian to younger orogenic ages.
Uses & applications
Used as crushed aggregate, dimension stone, landscaping rock, and decorative slabs; attractive pieces are collected but are generally not gem material.
Geological facts
The banding reflects metamorphic mineral segregation rather than ordinary sedimentary layering. Dark bands commonly contain biotite or amphibole, while pale bands are richer in quartz and feldspar.
Field identification & locations
Identify it by alternating light/dark bands and foliation, with visible grains and irregular splitting along the fabric; a streak test, hardness test, and thin section would distinguish gneiss from schist or banded slate. Common worldwide in ancient continental shields and mountain belts.
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