metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 (Mohs scale), Color: Pink, grey, tan, and white banding, Luster: Dull to pearly, Structure: Foliated with distinct bands (gneissic banding), Cleavage: Poor, SG: 2.6-2.9
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Pink, grey, tan, and white banding, Luster: Dull to pearly, Structure: Foliated with distinct bands (gneissic banding), Cleavage: Poor, SG: 2.6-2.9
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme pressure and temperature (typically deep in the crust). Ages can range from hundreds of millions to billions of years.
Uses & applications
Used as a dimension stone for flooring, Grave markers, facing stone on buildings, and crushed stone for road construction and landscaping.
Geological facts
Gneiss is some of the oldest rock found on Earth, with the Acasta Gneiss in Canada dated to approximately 4 billion years old. The name comes from a German word meaning 'sparkling'.
Field identification & locations
Identify by the characteristic alternating light and dark mineral bands (foliation). Common in the roots of mountain ranges and ancient continental shields like the Canadian Shield or the Appalachians.
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