metamorphic
Gneiss
Gneiss (Metamorphic high-grade)
Hardness: 6-7 (Mohs scale); Color: Gray, pink, or yellowish-brown with alternating dark and light bands; Luster: Dull to vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poor cleavage but tends to break along foliations; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 (Mohs scale)
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Gray, pink, or yellowish-brown with alternating dark and light bands; Luster: Dull to vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poor cleavage but tends to break along foliations; Specific gravity: 2.6-2.9.
Formation & geological history
Formed from the regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale) subjected to high temperatures and pressures deep within the Earth's crust. It often dates back to the Proterozoic or Archean eras in stable cratons.
Uses & applications
Used extensively in construction as crushed stone, dimension stone for flooring and facades, and as landscaping material. High-quality specimens with attractive banding are used for burial monuments and decorative tiles.
Geological facts
Gneiss is one of the oldest rocks on Earth, with the Acasta Gneiss in Canada dated at approximately 4.03 billion years old. It is the result of 'high-grade' metamorphism where minerals segregate into distinct bands without melting.
Field identification & locations
Identify in the field by its characteristic 'gneissic banding' where dark (mafic) minerals like biotite and light (felsic) minerals like quartz/feldspar form visible layers. Commonly found in mountain belts and glacial till. Collectors look for sharp contrast in color bands.
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