metamorphic
Gneiss
High-grade regional metamorphic rock
Hardness: 6-7 (Mohs), Color: gray and white bands with occasional brown streaks, Luster: dull to pearly, Crystal structure: coarse-grained with distinct foliation (gneissose banding), Cleavage: poor (planes of weakness along bands)
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Physical properties
Hardness: 6-7 (Mohs), Color: gray and white bands with occasional brown streaks, Luster: dull to pearly, Crystal structure: coarse-grained with distinct foliation (gneissose banding), Cleavage: poor (planes of weakness along bands)
Formation & geological history
Formed under high-pressure and high-temperature conditions deep in the Earth's crust, typically from granitic or sedimentary protoliths such as shale or sandstone during regional metamorphism.
Uses & applications
Used as a crushed stone in construction, road aggregates, dimension stone for flooring and building facades, and sometimes as landscaping rock.
Geological facts
Gneiss is one of the oldest rock types found on Earth. The Acasta Gneiss in Canada is approximately 4 billion years old, dating back to the Hadean Eon.
Field identification & locations
Identify by the characteristic alternating light and dark mineral bands (foliation). Found in mountains and cratons globally. Best identified by its lack of distinct cleavage compared to schist.
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