metamorphic
Banded gneiss
Gneiss, commonly quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectTypically medium-hard (Mohs ~6–7), gray to black with pale bands, granular texture, and vitreous to dull luster. Its hallmark is alternating mineral bands produced by foliation; composition commonly includes quartz, feldspar, and biotite or amphibole.
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Physical properties
Typically medium-hard (Mohs ~6–7), gray to black with pale bands, granular texture, and vitreous to dull luster. Its hallmark is alternating mineral bands produced by foliation; composition commonly includes quartz, feldspar, and biotite or amphibole.
Formation & geological history
Forms when granite or other preexisting rocks undergoes intense high-grade metamorphism, causing minerals to recrystallize and segregate into bands. Individual specimens may span broad Precambrian to younger ages, depending on locality.
Uses & applications
Used as crushed aggregate, dimension stone, paving, wall stone, and landscaping rock; attractive polished pieces are collected but are generally not gemstone material.
Geological facts
Gneissic banding can resemble sedimentary layering, but it reflects metamorphic mineral segregation and deformation. Dark bands are commonly mica- or amphibole-rich, while light bands are quartz- and feldspar-rich.
Field identification & locations
Identify it by coarse interlocking grains and strong light–dark banding; unlike shale, it does not split into thin flat sheets. Common in exposed mountain belts and glaciated terrains, including Canada, Scandinavia, the Appalachians, and the Alps.
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