metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray with dark bands; Luster: Dull to vitreous; Crystal structure: Foliated/Banded; Cleavage: Poor; Texture: Coarse-grained
- Hardness
- 6-7 on Mohs scale
- Color
- Gray with dark bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray with dark bands; Luster: Dull to vitreous; Crystal structure: Foliated/Banded; Cleavage: Poor; Texture: Coarse-grained
Formation & geological history
Formed by high-grade regional metamorphism of igneous (granite) or sedimentary (shale/sandstone) protoliths under extreme heat and pressure, typically during mountain-building events.
Uses & applications
Used as crushed stone in road construction, as decorative building stone, for flooring, and as curbing. Hand-rounded river pebbles are used in landscaping.
Geological facts
Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss in Canada is dated at approximately 4 billion years old. The term comes from the Middle High German word 'gneist', meaning 'spark'.
Field identification & locations
Identified by 'gneissose banding', where dark and light minerals separate into distinct layers without breaking easily along them. Commonly found in the roots of mountain ranges and shield areas.
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