metamorphic
Gneiss (likely banded granitic gneiss)
Quartz–feldspar–mica gneiss; commonly containing biotite and hornblende
AI-generated identification · may be incorrectTypically coarse-grained, with irregular light quartz/feldspar and dark biotite or hornblende bands; vitreous quartz, feldspar cleavage, and Mohs hardness about 6–7. The specimen appears weathered and rounded, with weakly visible compositional banding.
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Physical properties
Typically coarse-grained, with irregular light quartz/feldspar and dark biotite or hornblende bands; vitreous quartz, feldspar cleavage, and Mohs hardness about 6–7. The specimen appears weathered and rounded, with weakly visible compositional banding.
Formation & geological history
Formed when granite or other felsic rock underwent high-grade regional metamorphism, generally at gran. Temperatures and pressures deep in continental crust; many gneisses are Precambrian, though age depends on locality.
Uses & applications
Used as crushed aggregate, dimension stone, landscaping rock, and occasionally architectural facing; attractive pieces are collected but are rarely valuable as gemstones.
Geological facts
Gneiss commonly preserves evidence of intense deformation and partial melting, and its alternating bands distinguish it from more homogeneous granite.
Field identification & locations
Identify it by coarse interlocking crystals and light–dark mineral banding or streaking; common in continental shields and metamorphic mountain belts, including the Appalachians, Canadian Shield, Scandinavia, and Australia.
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