metamorphic
Banded Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Pink, gray, white, and black banding (foliation); Luster: Vitreous to dull; Crystal structure: Coarse-grained, crystalline; Cleavage: Poorly defined, breaks along irregular surfaces; Specific gravity: Approx. 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Pink, gray, white, and black banding (foliation)
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pink, gray, white, and black banding (foliation); Luster: Vitreous to dull; Crystal structure: Coarse-grained, crystalline; Cleavage: Poorly defined, breaks along irregular surfaces; Specific gravity: Approx. 2.6-2.9.
Formation & geological history
Formed through high-pressure, high-temperature regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks. The banding occurs due to recrystallization and migration of minerals into distinct layers. Many specimens date back to the Precambrian era, billions of years old.
Uses & applications
Used primarily as decorative stone in architecture, countertops, and flooring. In construction, it is used as crushed stone for road bases. Polished specimens are popular for landscaping and as educational/collection pieces.
Geological facts
Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The distinct 'gneissic banding' is a result of extreme tectonic forces that align minerals like feldspar, quartz, and mica.
Field identification & locations
Identify in the field by looking for 'segregation bands'—alternating light (feldspar/quartz) and dark (biotite/hornblende) minerals. Often found in mountainous regions or glacial deposits. Common in the Canadian Shield, Scandinavia, and the Appalachian Mountains.
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