metamorphic
Gneiss
Banded orthogneiss or paragneiss, typically quartz–feldspar–mica bearing
AI-generated identification · may be incorrectMedium- to coarse-grained, gray rock with wavy light and dark mineral bands; minerals show strong foliation but little obvious cleavage. Hardness is approximately 6–7 Mohs, with vitreous to dull luster and specific gravity around 2.6–2.8.
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Physical properties
Medium- to coarse-grained, gray rock with wavy light and dark mineral bands; minerals show strong foliation but little obvious cleavage. Hardness is approximately 6–7 Mohs, with vitreous to dull luster and specific gravity around 2.6–2.8.
Formation & geological history
Formed when granite or sedimentary rock underwent high-grade metamorphism involving intense heat and pressure, commonly during Precambrian to Paleozoic mountain-building events. The visible banding reflects mineral segregation and deformation.
Uses & applications
Widely used as dimension stone, landscaping stone, retaining walls, paving, and aggregate; attractive polished pieces can be used decoratively but it is not usually a gemstone.
Geological facts
Gneissic banding distinguishes it from more uniformly textured granite and finer-grained schist. Individual bands may fold, stretch, or contain quartz-rich veins.
Field identification & locations
Identify it by alternating pale quartz/feldspar and dark biotite or amphibole layers, plus a hard, crystalline texture; a fresh surface is most diagnostic. Common in ancient continental shields and metamorphic mountain belts worldwide.
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