metamorphic
Polished gneiss pebble
Quartz–feldspar–mica gneiss (exact mineral identification is uncertain from the image)
AI-generated identification · may be incorrectGray, white, and black banding with a polished, slightly lustrous surface; likely composed mainly of quartz and feldspar with dark mica or amphibole. Typical component hardness is about 6–7 Mohs; gneiss has no single crystal structure or consistent cleavage.
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Physical properties
Gray, white, and black banding with a polished, slightly lustrous surface; likely composed mainly of quartz and feldspar with dark mica or amphibole. Typical component hardness is about 6–7 Mohs; gneiss has no single crystal structure or consistent cleavage.
Formation & geological history
Formed when pre-existing rock was recrystallized and compositionally banded under high heat and pressure during regional metamorphism. Gneiss occurs in ancient continental crust and is commonly billions to hundreds of millions of years old.
Uses & applications
Used as decorative stone, landscaping aggregate, and sometimes in countertops or jewelry as polished pebbles.
Geological facts
The contrasting light and dark streaks are characteristic gneissic banding, formed as minerals segregated during metamorphism.
Field identification & locations
Look for irregular, wavy bands of light quartz/feldspar and darker minerals; the image shows a small, water-worn and polished specimen. Gneiss is widespread in continental mountain belts and glacial deposits.
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