metamorphic
Gneiss
Banded quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectTypically medium-to-coarse grained, with alternating light quartz/feldspar bands and dark biotite or amphibole; hardness about 6–7, vitreous to pearly luster, weak cleavage overall, and specific gravity around 2.6–2.8.
Identified More metamorphic →
Explore Gneiss in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Typically medium-to-coarse grained, with alternating light quartz/feldspar bands and dark biotite or amphibole; hardness about 6–7, vitreous to pearly luster, weak cleavage overall, and specific gravity around 2.6–2.8.
Formation & geological history
Formed by high-grade regional metamorphism of granite or sedimentary protoliths, commonly during Precambrian to Paleozoic mountain building. The visible streaking represents mineral segregation and tectonic deformation.
Uses & applications
Used as building stone, dimensional stone, paving, landscaping rock, and occasionally decorative slabs; collectible value is generally modest unless strongly banded or unusual.
Geological facts
Gneissic banding can resemble layering but is produced mainly by recrystallization and deformation. The dark protruding patch may be an amphibole-rich inclusion or weathered mafic mineral aggregate.
Field identification & locations
Identify it by alternating mineral bands, coarse interlocking grains, and resistance to scratching; quartz and feldspar are usually visible, unlike fine-grained slate. Common in ancient continental shields and deeply eroded mountain belts.
More like this
