metamorphic
Banded gneiss
Gneiss, commonly quartz–feldspar–biotite gneiss
AI-generated identification · may be incorrectTypically medium- to coarse-grained with alternating light quartz/feldspar and dark biotite or amphibole bands; vitreous to pearly luster, blocky fracture, and Mohs hardness about 6–7. The specimen’s strong foliation and compositional banding are characteristic, though exact minerals require testing.
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Physical properties
Typically medium- to coarse-grained with alternating light quartz/feldspar and dark biotite or amphibole bands; vitreous to pearly luster, blocky fracture, and Mohs hardness about 6–7. The specimen’s strong foliation and compositional banding are characteristic, though exact minerals require testing.
Formation & geological history
Formed by high-grade regional metamorphism that recrystallized and segregated a preexisting igneous or sedimentary rock under intense heat and pressure. Gneiss commonly records Precambrian to younger continental mountain-building events.
Uses & applications
Used as dimension stone, facing stone, paving, landscaping rock, and aggregate; attractive polished pieces are collected but are generally not used as gemstones.
Geological facts
Its banding is metamorphic, not ordinary sedimentary layering: minerals were stretched, recrystallized, and aligned during deformation. It can grade into migmatite where partial melting occurred.
Field identification & locations
Identify it by distinct light–dark mineral bands, coarse crystals, and foliation; unlike granite, the bands are usually visibly deformed or streaked. Common in ancient continental shields and metamorphic mountain belts worldwide.
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