metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 (Mohs scale), Color: Gray and white bands, Luster: Vitreous/Dull, Crystal structure: Coarse-grained/Foliated (Gneissic banding), Cleavage: Poorly developed.
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Gray and white bands, Luster: Vitreous/Dull, Crystal structure: Coarse-grained/Foliated (Gneissic banding), Cleavage: Poorly developed.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme pressure and temperature. Typical of Precambrian shields.
Uses & applications
Used as a building stone (facing stone), crushed stone for road construction, and occasionally for high-end countertops or cemetery monuments.
Geological facts
Gneiss is among the oldest known rocks on Earth. The Acasta Gneiss in Canada is approximately 4 billion years old.
Field identification & locations
Identified by 'Gneissic banding'—distinct light and dark colored mineral layers. Found globally in the roots of mountain ranges and ancient continental shields.
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