metamorphic
Gneiss (Granulite / Foliated Metamorphic Rock)
Gneiss
Hardness: 6-7 on the Mohs scale. Color: Off-white, grey, and cream with dark grey to black banding. Luster: Dull to vitreous. Texture: Medium to coarse-grained with characteristic gneissic banding (alternating light and dark mineral layers). Cleavage: Poor; fractures irregularly.
- Hardness
- 6-7 on the Mohs scale
- Color
- Off-white, grey, and cream with dark grey to black banding
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on the Mohs scale. Color: Off-white, grey, and cream with dark grey to black banding. Luster: Dull to vitreous. Texture: Medium to coarse-grained with characteristic gneissic banding (alternating light and dark mineral layers). Cleavage: Poor; fractures irregularly.
Formation & geological history
Formed by high-grade regional metamorphism under intense heat and pressure, recrystallizing pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) deep within Earth's crust.
Uses & applications
Commonly used as crushed stone for road construction, aggregate in concrete, dimension stone for landscaping, and building facings. Polished specimens are used in decorative crafts and rock collections.
Geological facts
Gneiss contains some of the oldest rock formations discovered on Earth, such as the Acasta Gneiss in Canada, which is approximately 4.03 billion years old.
Field identification & locations
Identified in the field by its distinctive alternating light (quartz/feldspar) and dark (biotite/amphibole) mineral bands that give it a striped or speckled appearance. Frequently found in riverbeds, glacial deposits, and mountain roots worldwide.
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