metamorphic
Gneiss
Gneiss
Hardness: 6-7 Mohs; Color: Gray, pink, brown, and white banding; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissose banding); Cleavage: Poor; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 Mohs
- Color
- Gray, pink, brown, and white banding
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Gray, pink, brown, and white banding; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissose banding); Cleavage: Poor; Specific Gravity: 2.6-2.9
Formation & geological history
Formed via high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths) under intense heat and pressure. Often found in the cores of mountain ranges and ancient continental shields.
Uses & applications
Used as a dimension stone for flooring, gravestones, and building facades; also used as crushed stone for road construction and landscaping. Highly durable and weather-resistant.
Geological facts
Gneiss is among the oldest known rocks on Earth, with the Acasta Gneiss in Canada dated to approximately 4 billion years ago. The distinctive stripes are caused by chemical segregation during metamorphism.
Field identification & locations
Identified by 'gneissose banding'—distinct light and dark layers that are often thick and irregular. Unlike schist, it does not split easily along layers. Commonly found in metamorphic terrains like the Appalachians or Alps.
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