metamorphic
Gneiss
Metamorphic Gneiss (High-grade regional metamorphism)
Hardness: 6-7 (Mohs scale); Color: Gray, black, and white with yellow-orange iron staining; Luster: Dull to slightly pearly; Crystal structure: Coarse-grained foliated texture (gneissic banding); Composition: Predominantly quartz, feldspar, and darker minerals like biotite or hornblende.
- Hardness
- 6-7 (Mohs scale)
- Color
- Gray, black, and white with yellow-orange iron staining
- Luster
- Dull to slightly pearly
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Gray, black, and white with yellow-orange iron staining; Luster: Dull to slightly pearly; Crystal structure: Coarse-grained foliated texture (gneissic banding); Composition: Predominantly quartz, feldspar, and darker minerals like biotite or hornblende.
Formation & geological history
Formed from the regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale) under intense heat and pressure deep within the Earth's crust. Most gneiss specimens date back to the Precambrian or Paleozoic eras.
Uses & applications
Used primarily in the construction industry as crushed stone for road bases or as 'granite' dimension stone for countertops and architectural cladding. Local specimens serve as great pieces for teaching geological layering.
Geological facts
Gneiss is among the oldest known rocks on Earth, with some specimens in the Acasta Gneiss complex dating back over 4 billion years. The banding occurs because minerals segregate into layers during high-pressure crystallization.
Field identification & locations
Identified by 'gneissic banding,' which is the segregation of light and dark colored minerals into distinguishable layers. Commonly found in mountainous regions or glacial deposits. Its rounded shape suggests it was weathered by water in a stream or glacial environment.
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