metamorphic
Banded gneiss (polished/tumbled cobble)
Gneiss, composed chiefly of quartz, feldspar, and mica/amphibole
AI-generated identification · may be incorrectTypically Mohs 6–7; gray to black with cream-pink bands, vitreous to pearly luster, and coarse recrystallized grains. Its diagnostic feature is compositional banding; cleavage is weak or absent as a whole, though mica may cleave.
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Physical properties
Typically Mohs 6–7; gray to black with cream-pink bands, vitreous to pearly luster, and coarse recrystallized grains. Its diagnostic feature is compositional banding; cleavage is weak or absent as a whole, though mica may cleave.
Formation & geological history
Formed by high-grade regional metamorphism of granite or sedimentary protoliths, causing minerals to segregate into alternating light and dark bands. Gneiss commonly forms in Precambrian continental crust, though younger examples also occur.
Uses & applications
Used as dimension stone, paving, landscaping rock, countertops, and decorative polished specimens; generally low jewelry value.
Geological facts
The stripes reflect metamorphic differentiation and intense deformation rather than ordinary sedimentary layering. Folded or contorted bands indicate substantial crustal pressure and temperature.
Field identification & locations
Identify it by hard, interlocking grains and persistent light-dark banding; quartz should scratch glass, while mica may appear as shiny flakes. Common worldwide in ancient mountain belts, including Scandinavia, Canada, India, Brazil, and the United States.
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