metamorphic
Gneiss
Banded quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectCoarse-grained, gray rock with pale banding and granular texture; typically quartz, feldspar, and biotite or hornblende. Hardness is about 6–7 Mohs, vitreous to dull luster, massive with strong compositional foliation and little cleavage as a whole.
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Physical properties
Coarse-grained, gray rock with pale banding and granular texture; typically quartz, feldspar, and biotite or hornblende. Hardness is about 6–7 Mohs, vitreous to dull luster, massive with strong compositional foliation and little cleavage as a whole.
Formation & geological history
Formed when granite or other high-grade crustal rocks were subjected to intense heat and pressure, causing mineral segregation into bands. It commonly formed during Precambrian to Paleozoic mountain-building events, though ages vary widely.
Uses & applications
Used as dimension stone, paving, walls, landscaping, aggregate, and monuments; attractive varieties are polished for architectural surfaces.
Geological facts
Gneissic banding records deformation and partial recrystallization deep in the continental crust. It may resemble granite, but its streaky mineral bands distinguish it.
Field identification & locations
Identify it by alternating light and dark mineral bands, coarse interlocking grains, and a hard, sparkly surface; quartzite and granite are common look-alikes. Common in ancient shields and metamorphic mountain belts worldwide.
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