metamorphic
Banded gneiss
Gneiss; typically quartz–feldspar–biotite ± amphibole
AI-generated identification · may be incorrectUsually gray, black, and cream compositional bands with a coarse, crystalline texture, vitreous to dull luster, and poor cleavage due to foliation. Hardness is about 6–7 Mohs; specific gravity is commonly 2.6–2.8.
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Physical properties
Usually gray, black, and cream compositional bands with a coarse, crystalline texture, vitreous to dull luster, and poor cleavage due to foliation. Hardness is about 6–7 Mohs; specific gravity is commonly 2.6–2.8.
Formation & geological history
Formed by high-grade regional metamorphism of granite, sedimentary rocks, or volcanic protoliths, causing mineral segregation and recrystallization. It may range from Precambrian to younger ages, depending on locality.
Uses & applications
Used as dimension stone, countertops, facing stone, paving, aggregate, and landscaping rock; attractive banded pieces are collected as specimens.
Geological facts
The alternating light and dark layers are mineral bands rather than sedimentary bedding, and may record intense deformation and deep-crustal metamorphism.
Field identification & locations
Identify it by strong, often folded light-dark banding and visible interlocking quartz/feldspar grains; unlike schist, it generally lacks abundant platy mica sheen. Common in ancient continental shields and metamorphic mountain belts.
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