metamorphic
Gneiss
Foliated quartz–feldspar gneiss, likely biotite-bearing
AI-generated identification · may be incorrectGray and white, strongly banded with dark mica-rich streaks and a coarse, crystalline texture. Typical minerals are quartz, feldspar, and biotite; hardness is about 6–7 Mohs, with no regular cleavage as a whole rock.
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Physical properties
Gray and white, strongly banded with dark mica-rich streaks and a coarse, crystalline texture. Typical minerals are quartz, feldspar, and biotite; hardness is about 6–7 Mohs, with no regular cleavage as a whole rock.
Formation & geological history
Forms when pre-existing rocks undergo high-grade heat and pressure, separating minerals into light and dark bands. Gneiss occurs in ancient continental crust, often billions of years old.
Uses & applications
Used as decorative and building stone, paving, aggregate, and as a collectible geological specimen.
Geological facts
The alternating mineral bands are called gneissic banding and distinguish it from many less strongly foliated metamorphic rocks.
Field identification & locations
Look for coarse, wavy or layered light quartz–feldspar bands alternating with darker mica-rich bands; common in ancient mountain belts and exposed continental shields. The specimen’s appearance is consistent with gneiss, though identification from a photo is not definitive.
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