metamorphic
Banded gneiss
Gneiss, commonly quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectTypically hard (Mohs 6–7), with alternating pink, tan, gray, and dark mineral bands; vitreous to dull luster and coarse-grained interlocking crystals. Foliation and compositional banding are prominent, with weak-to-variable cleavage.
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Physical properties
Typically hard (Mohs 6–7), with alternating pink, tan, gray, and dark mineral bands; vitreous to dull luster and coarse-grained interlocking crystals. Foliation and compositional banding are prominent, with weak-to-variable cleavage.
Formation & geological history
Formed by high-grade regional metamorphism of granite or sedimentary protoliths, causing recrystallization and mineral segregation; commonly Precambrian to younger. The specimen’s strongly curved bands may reflect folded gneissic layering.
Uses & applications
Used as dimension stone, landscaping rock, retaining walls, and aggregate; attractive polished pieces are collected, though it is not generally a gemstone.
Geological facts
The light bands are usually quartz and feldspar, while dark bands commonly contain biotite, amphibole, or garnet. Gneissic banding records intense heat, pressure, and deformation deep in Earth’s crust.
Field identification & locations
Identify it by coarse grains and alternating light–dark mineral bands, distinguishing it from finer-grained banded shale or slate. Common in ancient continental shields and exposed mountain belts, including Canada, Scandinavia, the eastern and western United States.
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