metamorphic
Banded gneiss pebble
Gneiss, commonly quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectHardness about 6–7 Mohs; rounded, fine-to-medium grained pebble with alternating white quartz/feldspar-rich bands and dark biotite- or amphibole-rich layers. Vitreous to dull luster, no obvious cleavage as a whole, and strong compositional foliation; exact minerals cannot be confirmed from the photo.
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Physical properties
Hardness about 6–7 Mohs; rounded, fine-to-medium grained pebble with alternating white quartz/feldspar-rich bands and dark biotite- or amphibole-rich layers. Vitreous to dull luster, no obvious cleavage as a whole, and strong compositional foliation; exact minerals cannot be confirmed from the photo.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent high-grade regional metamorphism, causing minerals to recrystallize and segregate into bands; many gneisses are Precambrian, though their source rocks may be younger. Common in continental shields, mountain belts, and glacial or river gravels.
Uses & applications
Used as durable decorative stone, landscaping aggregate, countertops, building stone, and a popular collector specimen; this rounded example is mainly ornamental.
Geological facts
The banding is metamorphic foliation rather than sedimentary layering, produced by intense heat, pressure, and deformation. Its polished-looking surface may result from natural abrasion in water or glacial transport.
Field identification & locations
Identify it by alternating light and dark mineral bands, crystalline texture, and lack of rounded sedimentary grains; distinguish it from banded marble, which fizzes with acid. Common in regions such as Scandinavia, Canada, the northeastern United States, India, and Australia.
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