metamorphic
Gneiss
Foliated Metamorphic Rock
Hardness: 6-7 (Mohs); Color: Pink, gray, white, and black bands; Luster: Vitreous to dull; Crystal structure: Coarse-grained phaneritic; Cleavage: Generally poor; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs)
- Color
- Pink, gray, white, and black bands
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs); Color: Pink, gray, white, and black bands; Luster: Vitreous to dull; Crystal structure: Coarse-grained phaneritic; Cleavage: Generally poor; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks at extreme temperatures and pressures. This leads to the segregation of minerals into bands (gneissic banding).
Uses & applications
Used primarily as dimension stone for counter tops, flooring, and gravestones. It is also used as crushed stone for road construction and landscaping.
Geological facts
The Acasta Gneiss in Canada is one of the oldest known crustal rocks on Earth, dated to approximately 4 billion years ago.
Field identification & locations
Identify in the field by looking for distinct mineral banding (foliation) where light-colored quartz/feldspar layers alternate with dark-colored mica/amphibole layers. Commonly found in mountainous regions or glacial deposits.
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