metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Banded grey, pink, cream, and black; Luster: Dull to vitreous; Crystal Structure: Foliated with granular textures; Cleavage: Poor; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Banded grey, pink, cream, and black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded grey, pink, cream, and black; Luster: Dull to vitreous; Crystal Structure: Foliated with granular textures; Cleavage: Poor; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale) subjected to intense heat and pressure. Many examples date back to the Precambrian era.
Uses & applications
Used as crushed stone for road construction, as dimension stone for architectural facades, floor tiles, and gravestones, and occasionally as decorative landscaping rock.
Geological facts
Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss in Canada is approximately 4.03 billion years old.
Field identification & locations
Identify by looking for 'gneissose banding,' which are distinct colored layers or streaks of minerals. Commonly found in the cores of mountain ranges and eroded crystalline shields.
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