metamorphic
Weathered gneissic metamorphic rock
Gneiss, likely quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectTypically 6–7 Mohs; tan, gray, and brown with strongly layered or streaked texture, dull to slightly vitreous luster, and no prominent cleavage as a whole. Bands reflect alternating quartz/feldspar-rich and mica-rich layers; the surface is heavily weathered and rounded.
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Physical properties
Typically 6–7 Mohs; tan, gray, and brown with strongly layered or streaked texture, dull to slightly vitreous luster, and no prominent cleavage as a whole. Bands reflect alternating quartz/feldspar-rich and mica-rich layers; the surface is heavily weathered and rounded.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent high-grade metamorphism, commonly during mountain building, at roughly าย 600–800°C and substantial pressure. The specimen’s exact geological age cannot be determined from the photograph; gneiss ranges from Archean to modern orogenic belts.
Uses & applications
Used as dimension stone, landscaping rock, riprap, aggregate, and occasionally polished architectural stone; generally limited jewelry value.
Geological facts
The coarse banding records mineral segregation during intense deformation and recrystallization. It may be confused with schist, but gneiss usually has thicker, more compositionally distinct bands and less obvious platy cleavage.
Field identification & locations
Look for alternating light quartz/feldspar bands and dark biotite or amphibole bands, coarse interlocking grains, and a hard, resistant surface. Common in ancient continental shields and metamorphic mountain belts, including Canada, Scandinavia, the Alps, and the Appalachians.
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