metamorphic
Gneiss
Foliated quartzo-feldspathic gneiss, commonly containing biotite or amphibole
AI-generated identification · may be incorrectTypically quartz and feldspar with dark biotite/amphibole bands; hardness about 6–7 Mohs, variable gray, pink, white, and black color, vitreous to pearly luster, and weak cleavage inherited from mica. The specimen shows coarse, irregular compositional banding and rounded stream-worn surfaces.
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Physical properties
Typically quartz and feldspar with dark biotite/amphibole bands; hardness about 6–7 Mohs, variable gray, pink, white, and black color, vitreous to pearly luster, and weak cleavage inherited from mica. The specimen shows coarse, irregular compositional banding and rounded stream-worn surfaces.
Formation & geological history
Formed by high-grade regional metamorphism of granite or sedimentary protoliths under intense heat and pressure, commonly during Precambrian to Paleozoic mountain-building events. Exact age cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, road stone, dimension stone, landscaping material, and occasionally decorative slabs; generally little jewelry value.
Geological facts
Gneiss is recognized by alternating light quartz-feldspar and dark mafic/mineral bands, unlike the more evenly granular texture of granite. Some gneisses preserve evidence of ancient continental crust and major tectonic collisions.
Field identification & locations
Common in continental shield regions and mountain belts, including Canada, the northeastern and western United States, Scandinavia, India, and Australia. Look for mineral banding, coarse grains, and resistance to scratching by steel; a field test cannot reliably distinguish it from banded granite without thin-section or chemical analysis.
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