metamorphic
Banded gneiss
Gneiss, typically quartz–feldspar–mica assemblage
AI-generated identification · may be incorrectMedium- to coarse-grained, strongly foliated rock with alternating pale quartz/feldspar and dark mica or amphibole bands; hardness about 6–7 Mohs, vitreous to dull luster, negligible cleavage as a whole, and specific gravity roughly 2.6–2.8.
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Physical properties
Medium- to coarse-grained, strongly foliated rock with alternating pale quartz/feldspar and dark mica or amphibole bands; hardness about 6–7 Mohs, vitreous to dull luster, negligible cleavage as a whole, and specific gravity roughly 2.6–2.8.
Formation & geological history
Formed by high-grade regional metamorphism of granite or sedimentary protoliths, commonly during Precambrian to Paleozoic mountain-building; intense heat and pressure segregated minerals into compositional bands.
Uses & applications
Used as dimension stone, paving, landscaping stone, countertops, and decorative slabs; attractive pieces have modest collector value.
Geological facts
The specimen’s planar banding and foliation distinguish gneiss from massive granite, while its layered appearance may superficially resemble schist.
Field identification & locations
Identify it by alternating light and dark mineral bands, granular texture, and streak-free hardness; common in ancient continental shields and metamorphic mountain belts worldwide, including Canada, Scandinavia, India, Brazil, and the United States.
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