metamorphic
Pink-and-black gneiss
Foliated granitic gneiss, commonly quartz–feldspar–biotite gneiss
AI-generated identification · may be incorrectTypically composed of pink potassium feldspar, gray/clear quartz, and black biotite or amphibole in visible bands; hardness about 6–7 Mohs, granular texture, vitreous to dull luster, and no prominent cleavage as a whole.
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Physical properties
Typically composed of pink potassium feldspar, gray/clear quartz, and black biotite or amphibole in visible bands; hardness about 6–7 Mohs, granular texture, vitreous to dull luster, and no prominent cleavage as a whole.
Formation & geological history
Forms when granite or other felsic rock undergoes intense regional metamorphism, causing mineral segregation into alternating light and dark bands. Many examples are Precambrian, commonly older than 1 billion years.
Uses & applications
Used as dimension stone, countertops, flooring, landscaping rock, monuments, and decorative polished specimens; generally modest collector value.
Geological facts
The banding is metamorphic foliation, not sedimentary layering. Pink coloration usually indicates potassium-rich feldspar, while dark streaks reflect mica or amphibole.
Field identification & locations
Identify it by coarse interlocking grains and irregular, wavy light–dark banding; granite lacks strong foliation, whereas schist is usually more flaky. Common in ancient mountain belts and exposed continental shields.
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