metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Pink, gray, white, and black banding; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated; Cleavage: Poor; Specific gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Pink, gray, white, and black banding
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pink, gray, white, and black banding; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated; Cleavage: Poor; Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary (shale/sandstone) protoliths under extreme pressure and temperature. Most are Precambrian or Paleozoic in age.
Uses & applications
Used in construction as crushed stone, dimension stone for flooring and facades, and as landscaping material. Polished slabs are used for countertops.
Geological facts
Gneiss is characterized by its distinct light and dark alternating bands, a phenomenon called gneissic banding. Some of the oldest rocks on Earth, the Acasta Gneiss, are estimated to be 4.03 billion years old.
Field identification & locations
Identify by looking for 'segregated' bands of different minerals. Commonly found in most continental shields and mountainous regions. Collectors look for unique banding patterns or inclusions like garnets.
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