metamorphic
Polished banded gneiss pebble
Gneiss, chiefly quartz–feldspar–mica; exact mineralogy uncertain from image
AI-generated identification · may be incorrectHardness about 6–7 Mohs; gray, black, and white, with a matte-to-satin polish and strong compositional banding. Granoblastic crystalline texture; no obvious cleavage as a whole rock, though mica layers may split.
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Physical properties
Hardness about 6–7 Mohs; gray, black, and white, with a matte-to-satin polish and strong compositional banding. Granoblastic crystalline texture; no obvious cleavage as a whole rock, though mica layers may split.
Formation & geological history
Formed by high-grade metamorphism of granite or sedimentary protoliths, typically during Precambrian to Paleozoic mountain-building; the rounded form is likely water-worn and then polished.
Uses & applications
Used as a decorative stone, cabochon, paperweight, and lapidary specimen; larger blocks may serve as dimension stone or aggregate.
Geological facts
The light bands are commonly quartz and feldspar, while dark bands are biotite, amphibole, or other ferromagnesian minerals. Gneissic banding records mineral segregation under intense heat and pressure.
Field identification & locations
Identify it by alternating light and dark mineral bands, coarse interlocking grains, and greater hardness than most sedimentary rocks; common in ancient continental shields and glacial or river gravels.
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