metamorphic
Gneiss
Gneiss (Foliated Metamorphic Rock)
Hardness: 6-7 on Mohs scale. Color: Alternating light (quartz/feldspar) and dark (biotite/amphibole) bands, with reddish-brown iron oxide weathering staining. Luster: Dull to earthy weathered surface, vitreous on fresh breaks. Texture: Medium to coarse-grained with prominent gneissic banding.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to earthy weathered surface, vitreous on fresh breaks
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Alternating light (quartz/feldspar) and dark (biotite/amphibole) bands, with reddish-brown iron oxide weathering staining. Luster: Dull to earthy weathered surface, vitreous on fresh breaks. Texture: Medium to coarse-grained with prominent gneissic banding.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing sedimentary (shale/sandstone) or igneous (granite) protoliths under extreme temperature and pressure deep within the Earth crust.
Uses & applications
Used as crushed stone in construction, road base, aggregate, decorative landscaping rock, and occasionally as dimension stone or building facing.
Geological facts
Gneiss is one of the oldest rock types found on Earth. The Acasta Gneiss in the Northwest Territories, Canada, is estimated to be approximately 4.03 billion years old, making it one of the oldest known intact crustal fragments.
Field identification & locations
Identified in the field by distinct, parallel light and dark mineral banding (gneissic foliation) and high hardness. Commonly found in exposed Precambrian shield areas, mountain belts, and glaciated riverbeds or fields.
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