metamorphic
Banded gneiss
Gneiss, commonly quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectFine- to medium-grained, pale gray rock with thin, nearly parallel light and dark bands; typically composed of quartz, feldspar, and mica. Hardness is about 6–7 Mohs, with a dull to slightly glassy luster and weak foliation rather than distinct cleavage.
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Physical properties
Fine- to medium-grained, pale gray rock with thin, nearly parallel light and dark bands; typically composed of quartz, feldspar, and mica. Hardness is about 6–7 Mohs, with a dull to slightly glassy luster and weak foliation rather than distinct cleavage.
Formation & geological history
Formed when granite or other rocks undergo high-temperature, high-pressure metamorphism, causing minerals to recrystallize and segregate into bands. Individual specimens may represent Precambrian to younger continental crust, but age cannot be determined from the photograph.
Uses & applications
Used as dimension stone, architectural facing, paving, aggregate, and landscape rock; attractive polished pieces are collected but usually have modest value.
Geological facts
The banding is metamorphic, not sedimentary layering, and records deformation and mineral separation deep in the crust.
Field identification & locations
Identify it by alternating quartz/feldspar-rich light bands and darker biotite- or amphibole-rich bands, plus a hard, crystalline texture. Common in ancient continental shields and mountain belts worldwide; an unpolished hand specimen is typically worth $1–$15, while unusual or polished specimens may bring $20–$100.
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