metamorphic
Banded Gneiss
Foliated Gneiss
Hardness: 6-7 on Mohs scale; Color: Alternating dark (mafic) and light (felsic) bands; Luster: Vitreous to dull; Crystal structure: Coarse-grained, crystalline; Cleavage: Poor, but splits along foliation layers; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Alternating dark (mafic) and light (felsic) bands
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Alternating dark (mafic) and light (felsic) bands; Luster: Vitreous to dull; Crystal structure: Coarse-grained, crystalline; Cleavage: Poor, but splits along foliation layers; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme heat and pressure, often at deep crustal levels during mountain-building events.
Uses & applications
Used as a decorative stone in construction (countertops, flooring), as crushed stone for road aggregate, and as polished pocket stones or landscaping specimens for collectors.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The banding, known as 'gneissose banding', is caused by the segregation of minerals during intense pressure.
Field identification & locations
Identified in the field by its distinct 'salt and pepper' striped appearance and lack of schistosity (it does not split as easily as schist). Found globally in continental shields and mountain ranges like the Appalachians or Himalayas.
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