metamorphic
Banded gneiss
Quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectTypically coarse-grained with alternating light quartz/feldspar and darker biotite-rich bands; hardness about 6–7, vitreous to pearly luster, and no prominent cleavage in the rock. The specimen shows foliation and compositional banding, though its exact minerals are best confirmed microscopically.
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Physical properties
Typically coarse-grained with alternating light quartz/feldspar and darker biotite-rich bands; hardness about 6–7, vitreous to pearly luster, and no prominent cleavage in the rock. The specimen shows foliation and compositional banding, though its exact minerals are best confirmed microscopically.
Formation & geological history
Forms when granite or other crustal rock undergoes high-grade regional metamorphism, commonly during mountain building; many examples are Precambrian (older than 541 million years), though younger gneisses occur worldwide.
Uses & applications
Used as dimension stone, architectural facing, paving, landscaping material, and occasionally as a decorative polished stone; ordinary hand specimens have modest collector value.
Geological facts
Gneissic banding results from recrystallization and segregation of minerals under intense heat and pressure, not from simple sediment layering.
Field identification & locations
Identify it by alternating light and dark mineral bands, coarse interlocking grains, and foliation; common sources include ancient continental shields and metamorphic mountain belts. A fresh surface or streak testing can help distinguish it from banded granite or schist.
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