metamorphic

Polished banded gneiss pebble

Gneiss, commonly quartz–feldspar–biotite gneiss

AI-generated identification · may be incorrect

Hardness about 6–7 Mohs; gray with pale quartz/feldspar and darker biotite-rich bands, vitreous to dull luster, and elongated compositional banding rather than cleavage. The smooth oval shape indicates tumbling or water erosion; exact mineral proportions cannot be confirmed from the image.

Identified September 19, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when preexisting igneous or sedimentary rock underwent intense high-grade metamorphism, typically during mountain-building, with mineral segregation into gneissic bands. Such rocks are commonly Precambrian to Paleozoic, though age depends on its source locality.

Uses & applications

Used as decorative stone, polished specimens, cabochons, landscaping material, and occasionally dimension stone. It has modest collector value unless the banding or provenance is exceptional.

Geological facts

The stripes are compositional layers produced by metamorphic recrystallization and deformation, not ordinary sedimentary bedding. Gneiss commonly contains quartz, feldspar, mica, amphibole, or garnet.

Field identification & locations

Identify it by strong light–dark banding, interlocking crystalline texture, and inability to scratch it with a steel knife; quartz-rich areas may scratch glass. Common in ancient continental shields and metamorphic mountain belts.