metamorphic
Granite Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Pink, gray, and black banding; Luster: Dull to vitreous; Crystal structure: Coarse-grained phaneritic/foliated; Cleavage: Poor, but splits along bands; Specific gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Pink, gray, and black banding
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pink, gray, and black banding; Luster: Dull to vitreous; Crystal structure: Coarse-grained phaneritic/foliated; Cleavage: Poor, but splits along bands; Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of a granitic protolith under extreme pressure and temperature. This process causes minerals like quartz, feldspar, and mica to segregate into visible bands.
Uses & applications
Used in heavy construction, road aggregates, floor tiles, and as decorative landscaping stones due to its durability and aesthetic banding.
Geological facts
The banding in gneiss is called gneissic foliation. It is one of the oldest rock types on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years.
Field identification & locations
Identify by looking for distinct alternating dark and light colored mineral bands. It is commonly found in the roots of mountain ranges (crystalline basements). Collectors should look for sharp color contrast between the pink orthoclase and black biotite.
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