metamorphic
Banded gneiss
Gneiss, commonly quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectTypically medium-to-coarse grained, with alternating light quartz/feldspar bands and dark biotite or hornblende layers; Mohs hardness about 6–7, vitreous to pearly luster, and weak foliation rather than true cleavage. The specimen is gray, strongly banded, and appears possibly weathered or lightly polished.
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Physical properties
Typically medium-to-coarse grained, with alternating light quartz/feldspar bands and dark biotite or hornblende layers; Mohs hardness about 6–7, vitreous to pearly luster, and weak foliation rather than true cleavage. The specimen is gray, strongly banded, and appears possibly weathered or lightly polished.
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. It occurs in continental basement and metamorphic belts.
Uses & applications
Used as crushed aggregate, building stone, paving, countertops, monuments, and decorative slabs; attractive banded pieces are collected but are rarely highly valuable.
Geological facts
Gneissic banding results from mineral segregation and deformation, not ordinary sedimentary layering. Some gneisses preserve evidence of ancient continental crust and can be billions of years old.
Field identification & locations
Identify it by alternating light and dark mineral bands, visible interlocking grains, and a hard, massive texture; it differs from slate or schist by its coarser grains and less fissile fabric. Common worldwide in shield regions and mountain belts.
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