metamorphic
Gneiss
High-grade metamorphic rock (Gneiss)
Hardness: 6-7 on Mohs scale; Color: Gray with tan/brown bands; Luster: Dull to sub-vitreous; Structure: Foliated with distinct banding (gneissic banding); Cleavage: Poor.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray with tan/brown bands
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray with tan/brown bands; Luster: Dull to sub-vitreous; Structure: Foliated with distinct banding (gneissic banding); Cleavage: Poor.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense heat and pressure deep within the Earth's crust. Most specimens date back hundreds of millions to billions of years.
Uses & applications
Used as a crushed stone for construction, road base, and occasionally as a decorative stone or dimension stone for landscaping.
Geological facts
Gneiss is one of the oldest known rock types on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The banding is caused by the segregation of light minerals (quartz/feldspar) and dark minerals (biotite/amphibole).
Field identification & locations
Identified by its characteristic alternating bands of different mineral colors. Commonly found in the cores of mountain ranges or in glacial tills. Hand-sized river cobbles like this are common in areas with metamorphic bedrock.
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