metamorphic
Banded gneiss pebble
Gneiss, commonly quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectTypically 6–7 Mohs; gray to white with dark mineral bands, vitreous to dull luster, granular interlocking crystals, and no consistent cleavage. The specimen appears water-worn and may show quartz, feldspar, and biotite/hornblende layers, though exact identification requires testing.
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Physical properties
Typically 6–7 Mohs; gray to white with dark mineral bands, vitreous to dull luster, granular interlocking crystals, and no consistent cleavage. The specimen appears water-worn and may show quartz, feldspar, and biotite/hornblende layers, though exact identification requires testing.
Formation & geological history
Formed when granite or other felsic rock undergoes high-grade metamorphism, producing compositional banding through recrystallization and mineral segregation. Gneiss commonly records Precambrian to younger continental-crust metamorphism.
Uses & applications
Used as crushed aggregate, dimension stone, paving, and landscaping rock; attractive polished pieces are collected but are not usually jewelry gemstones.
Geological facts
Its alternating light and dark bands distinguish it from massive granite, while strongly folded bands indicate intense deformation. The rounded shape reflects stream or beach abrasion rather than its original formation.
Field identification & locations
Look for coarse interlocking grains and discontinuous light–dark banding; a glassy quartz-rich streak and hardness near 7 support the identification. Common in continental mountain belts and glacial gravels worldwide.
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