metamorphic
Banded gneiss
Gneiss, commonly quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectTypically hard (Mohs 6–7), gray to brown, with alternating pale quartz/feldspar and darker biotite or amphibole bands. It has a granular crystalline texture, variable foliation, and no prominent cleavage; the image shows strongly folded compositional banding.
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Physical properties
Typically hard (Mohs 6–7), gray to brown, with alternating pale quartz/feldspar and darker biotite or amphibole bands. It has a granular crystalline texture, variable foliation, and no prominent cleavage; the image shows strongly folded compositional banding.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent high-grade metamorphism, commonly at roughly 600–800°C during mountain building. Its exact geological age cannot be determined from the image; gneiss ranges from Archean to modern orogenic belts.
Uses & applications
Used as dimension stone, facing stone, paving, landscaping rock, and aggregate; attractive polished pieces may be sold decoratively but are generally low-value.
Geological facts
The curved bands are mineral segregation and deformation structures called gneissic banding, recording intense ductile compression and recrystallization. It may resemble banded granite, but its layers are typically more continuous and visibly foliated.
Field identification & locations
Identify it by alternating light and dark mineral bands, coarse grains, and a massive, very hard surface; a fresh break reveals quartz, feldspar, and dark mica. Common worldwide in ancient continental shields and metamorphic mountain belts.
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