metamorphic
Rounded granitic gneiss pebble
Quartz–feldspar–biotite gneiss (tentative; exact identification requires closer inspection)
AI-generated identification · may be incorrectCoarse, mottled gray-brown rock with pale quartz/feldspar and darker mineral grains; the photo suggests weak foliation or banding. Typical gneiss is Mohs 6–7, with a granular crystalline texture and no regular cleavage as a whole rock.
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Physical properties
Coarse, mottled gray-brown rock with pale quartz/feldspar and darker mineral grains; the photo suggests weak foliation or banding. Typical gneiss is Mohs 6–7, with a granular crystalline texture and no regular cleavage as a whole rock.
Formation & geological history
Formed when pre-existing rock was recrystallized under high heat and pressure, producing gneissic texture; its age cannot be determined from appearance. The rounded shape likely reflects prolonged erosion and transport in water.
Uses & applications
A pebble of this size is mainly decorative or collectible; similar gneiss is used as dimension stone, aggregate, and landscaping rock.
Geological facts
Gneiss is identified by mineral segregation or banding, though weathering can obscure it and make it resemble granite.
Field identification & locations
The specimen was found near Bahia, Brazil, where ancient crystalline basement rocks make quartz–feldspar metamorphic rocks plausible. Check a fresh surface for interlocking grains and any compositional bands; a photo alone cannot distinguish gneiss confidently from weathered granite.
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