metamorphic
Banded gneiss
Gneiss, chiefly quartz–feldspar–mica (variable composition)
AI-generated identification · may be incorrectTypically medium-to-coarse grained with alternating light quartz/feldspar and dark biotite or amphibole bands; Mohs hardness about 6–7, vitreous to dull luster, weak cleavage overall, and no regular crystal shape. The specimen shows folded, streaky mineral banding and weathered tan surfaces.
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Physical properties
Typically medium-to-coarse grained with alternating light quartz/feldspar and dark biotite or amphibole bands; Mohs hardness about 6–7, vitreous to dull luster, weak cleavage overall, and no regular crystal shape. The specimen shows folded, streaky mineral banding and weathered tan surfaces.
Formation & geological history
Forms when granite or other preexisting rocks undergo intense regional metamorphism, recrystallization, and deformation at high temperature and pressure, commonly deep within mountain belts. Individual specimens may range from Precambrian to younger Phanerozoic ages.
Uses & applications
Used as dimension stone, paving, facing stone, aggregate, and landscape rock; attractive polished pieces are collected but are generally not gemstone material.
Geological facts
The characteristic compositional banding records extreme crustal deformation and mineral segregation. Gneiss is pronounced “nice.”
Field identification & locations
Identify it by coarse interlocking grains and distinct light-dark banding, unlike finer-grained slate or schist; quartz should scratch glass, while feldspar commonly shows blocky cleavage. Common worldwide in exposed Precambrian shields and metamorphic mountain cores.
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