metamorphic
Banded gneiss
Gneiss, commonly quartz–feldspar–mica bearing
AI-generated identification · may be incorrectTypically medium- to coarse-grained with alternating light quartz/feldspar and dark biotite or amphibole bands; Mohs hardness about 6–7, vitreous to dull luster, and weak cleavage inherited from mica. The specimen shows strong foliation and compositional banding, though exact identification from an image is approximate…
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Physical properties
Typically medium- to coarse-grained with alternating light quartz/feldspar and dark biotite or amphibole bands; Mohs hardness about 6–7, vitreous to dull luster, and weak cleavage inherited from mica. The specimen shows strong foliation and compositional banding, though exact identification from an image is approximate.
Formation & geological history
Formed by high-grade regional metamorphism of granite or other rocks, with heat and pressure segregating minerals into bands. Gneiss commonly records Precambrian to younger mountain-building events and is widespread in continental crust.
Uses & applications
Used as dimension stone, building and paving aggregate, landscaping rock, and occasionally as decorative slabs. Attractive banded pieces are collected but generally have modest value.
Geological facts
Gneiss differs from schist by its coarse, compositional banding and relatively massive texture rather than abundant aligned mica. Some gneisses preserve evidence of ancient continental crust and deep burial.
Field identification & locations
Look for alternating pale and dark mineral bands, interlocking grains, and a hard, non-fissile texture; it commonly occurs in mountain belts and glacial gravel deposits. The surrounding pieces appear like landscaping or glacial gravel, so provenance cannot be determined visually.
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