metamorphic
Gneiss
Felsic banded gneiss, chiefly quartz–feldspar–mica
AI-generated identification · may be incorrectTypically 6–7 Mohs; coarse-grained with alternating light quartz/feldspar bands and dark biotite/amphibole, displaying vitreous to pearly luster and strong compositional foliation. The image shows folded, strongly banded structure and weathered iron-oxide staining.
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Physical properties
Typically 6–7 Mohs; coarse-grained with alternating light quartz/feldspar bands and dark biotite/amphibole, displaying vitreous to pearly luster and strong compositional foliation. The image shows folded, strongly banded structure and weathered iron-oxide staining.
Formation & geological history
Formed by high-grade regional metamorphism of granite or sedimentary protoliths under elevated temperature and pressure, commonly during Precambrian mountain-building; individual specimens may be hundreds of millions to billions of years old.
Uses & applications
Used as dimension stone, building and paving stone, crushed aggregate, monuments, and landscaping rock; attractive polished pieces are collected but are generally less valuable than gem material.
Geological facts
Gneissic banding results from mineral segregation and deformation, and folds can preserve multiple episodes of crustal metamorphism. It commonly grades into migmatite where partial melting begins.
Field identification & locations
Identify it by coarse alternating light/dark bands, abundant visible feldspar and quartz, and foliation that differs from the more uniform texture of granite. Common worldwide in ancient continental shields and exposed mountain belts.
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