metamorphic

Gneiss

Gneiss; quartz–feldspar–mica metamorphic rock

AI-generated identification · may be incorrect

Typically medium-to-coarse grained, with interleaved light quartz/feldspar and dark biotite or amphibole; the image shows strongly folded, swirled gneissic banding. Hardness is about 6–7, luster granular to slightly glassy, with weak cleavage inherited from mica and specific gravity around 2.6–2.8.

Identified September 12, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when granite, shale, or other protoliths undergo high-grade regional metamorphism, commonly during continental collision; individual examples may be Precambrian to younger.

Uses & applications

Used as building stone, dimension stone, paving, aggregate, and decorative slabs; attractive figured varieties are collected or used for countertops.

Geological facts

The contorted bands record intense ductile deformation and partial mineral segregation during metamorphism, often producing migmatitic or augen textures.

Field identification & locations

Identify it by compositional banding, coarse interlocking grains, and resistance to scratching; common in ancient continental shields and deeply exposed mountain belts worldwide. A thin section or laboratory analysis may be needed to distinguish it from migmatite or strongly foliated granite.