metamorphic
Banded gneiss (likely orthogneiss or paragneiss)
Gneiss; composition commonly quartz + feldspar + mica, with possible amphibole/biotite
AI-generated identification · may be incorrectCoarse-grained, strongly banded gray, cream, brown, and rusty layers; vitreous quartz and dull feldspar with micaceous streaks. Hardness about 6–7 Mohs, nonmetallic luster, no consistent cleavage as a whole, and commonly high specific gravity around 2.6–2.8.
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Physical properties
Coarse-grained, strongly banded gray, cream, brown, and rusty layers; vitreous quartz and dull feldspar with micaceous streaks. Hardness about 6–7 Mohs, nonmetallic luster, no consistent cleavage as a whole, and commonly high specific gravity around 2.6–2.8.
Formation & geological history
Formed when granite or sedimentary rock underwent intense high-grade metamorphism, causing minerals to recrystallize and segregate into bands. It may be hundreds of millions to billions of years old, depending on its source terrain.
Uses & applications
Used as dimension stone, architectural facing, paving, crushed aggregate, and landscaping rock; attractive polished pieces are collected but are generally not gemstone grade.
Geological facts
Its alternating light and dark bands record metamorphic mineral segregation and deformation. Rust-colored zones commonly reflect iron-bearing minerals or weathering along fractures.
Field identification & locations
Identify it by coarse interlocking grains and compositional banding, unlike flat sedimentary bedding; quartz-rich layers resist scratching, while mica may peel or sparkle. Common in ancient continental shields and metamorphic mountain belts worldwide.
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