metamorphic
Banded gneiss pebble
Gneiss, typically quartz–feldspar–mica banded rock
AI-generated identification · may be incorrectHardness about 6–7 Mohs; gray-brown with contorted light and dark mineral bands, dull to weakly lustrous, granular crystalline texture, and no obvious cleavage as a whole. Bands reflect alternating quartz/feldspar-rich and biotite/amphibole-rich layers.
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Physical properties
Hardness about 6–7 Mohs; gray-brown with contorted light and dark mineral bands, dull to weakly lustrous, granular crystalline texture, and no obvious cleavage as a whole. Bands reflect alternating quartz/feldspar-rich and biotite/amphibole-rich layers.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent high-grade metamorphism, causing mineral segregation and folding; protolith ages vary widely, commonly Precambrian to Paleozoic. The rounded shape indicates prolonged stream or beach abrasion.
Uses & applications
Used mainly as a decorative, educational, or collectible stone; larger gneiss is used for building stone, paving, countertops, and crushed aggregate, but this pebble has little commercial utility.
Geological facts
The swirling banding records intense ductile deformation deep in Earth’s crust. It can resemble migmatite or strongly folded schist, but the relatively coarse, competent banding favors gneiss.
Field identification & locations
Common in glacial deposits, mountain belts, river gravels, and rocky shorelines worldwide; identify by hard, interlocking grains and compositional banding that remains visible across the rounded surface. A hand lens can reveal quartz, feldspar, and dark mica.
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