metamorphic
Banded gneiss
Orthogneiss or paragneiss, chiefly quartz–feldspar–mica
AI-generated identification · may be incorrectTypically gray, tan, and black with compositional banding, foliation, and quartz/feldspar-rich layers; minerals have a glassy to pearly luster. Hardness is about 6–7 Mohs, with no consistent cleavage as a whole and generally high specific gravity (2.6–2.8).
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Physical properties
Typically gray, tan, and black with compositional banding, foliation, and quartz/feldspar-rich layers; minerals have a glassy to pearly luster. Hardness is about 6–7 Mohs, with no consistent cleavage as a whole and generally high specific gravity (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. Individual examples may be Precambrian to younger, depending on locality.
Uses & applications
Used as crushed aggregate, dimension stone, paving, walls, and decorative slabs; attractive pieces are collected or polished, though this specimen appears ordinary rather than gem-grade.
Geological facts
The alternating light and dark bands record metamorphic deformation and mineral segregation, not ordinary sedimentary layering. Folded or disrupted bands indicate strong tectonic pressure.
Field identification & locations
Identify it by coarse interlocking grains and persistent light–dark foliation; quartz and feldspar commonly dominate pale bands, while biotite or amphibole darkens others. Common in ancient continental shields and exposed mountain belts worldwide.
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