metamorphic
Rounded gneiss pebble
Banded gneiss; commonly quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectTypically gray, brown, and black with faint compositional banding; granular texture and dull to subtly sparkling luster. Hardness is about 6–7 Mohs, with poor/irregular cleavage and specific gravity around 2.6–2.8.
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Physical properties
Typically gray, brown, and black with faint compositional banding; granular texture and dull to subtly sparkling luster. Hardness is about 6–7 Mohs, with poor/irregular cleavage and specific gravity around 2.6–2.8.
Formation & geological history
Formed when pre-existing granite or sedimentary rock underwent intense regional metamorphism, recrystallizing quartz, feldspar, and mica into bands during mountain building. Individual gneiss bodies may be Precambrian to much younger; this rounded specimen was shaped by erosion and transport.
Uses & applications
Used as crushed stone, road aggregate, dimension stone, landscaping rock, and occasionally decorative slabs. Common pebbles have little commercial value.
Geological facts
The alternating light and dark bands reflect mineral segregation under high pressure and temperature, rather than ordinary sedimentary layering.
Field identification & locations
Identify it by its hard, speckled surface and irregular light–dark banding; quartz and feldspar resist scratching while mica may glitter or flake. Common in glacial deposits, streambeds, and eroded mountain terrains worldwide.
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