metamorphic
Banded gneissic rock
Gneiss, likely quartz–feldspar–biotite gneiss
AI-generated identification · may be incorrectCoarse-grained, strongly foliated with alternating pale and dark bands; likely Mohs hardness 6–7, vitreous to dull luster, negligible cleavage as a whole, and specific gravity about 2.6–2.8. The rounded surface appears naturally weathered or water-worn.
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Physical properties
Coarse-grained, strongly foliated with alternating pale and dark bands; likely Mohs hardness 6–7, vitreous to dull luster, negligible cleavage as a whole, and specific gravity about 2.6–2.8. The rounded surface appears naturally weathered or water-worn.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent intense high-grade metamorphism, causing mineral segregation into compositional bands. Such rocks commonly formed during Precambrian to Paleozoic mountain-building events, though age cannot be determined from the image.
Uses & applications
Used as crushed stone, road aggregate, landscaping rock, building stone, and decorative polished slabs; attractive pieces may be sold as display specimens.
Geological facts
The banding records directed pressure and partial recrystallization deep in the crust. Gneiss commonly contains quartz, feldspar, mica, amphibole, or garnet.
Field identification & locations
Identify by alternating light quartz–feldspar and dark mica/amphibole bands, crystalline texture, and resistance to scratching; unlike layered sedimentary rock, the bands are metamorphic mineral segregation. Common in ancient continental shields and metamorphic mountain belts.
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