metamorphic
Gneiss (River Cobble)
Gneiss
Hardness: 6-7 Mohs; Color: Alternating bands of light and dark minerals; Luster: Dull to slightly vitreous; Structure: Foliated (gneissic banding); Cleavage: None, but breaks along planes of weakness.
- Hardness
- 6-7 Mohs
- Color
- Alternating bands of light and dark minerals
- Luster
- Dull to slightly vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Alternating bands of light and dark minerals; Luster: Dull to slightly vitreous; Structure: Foliated (gneissic banding); Cleavage: None, but breaks along planes of weakness.
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary protoliths. It underwent intense heat and pressure, causing mineral recrystallization and the segregation of minerals into distinct bands.
Uses & applications
Commonly used as crushed stone in construction, road base, and occasionally as decorative landscaping stones or stone veneer.
Geological facts
Gneiss is one of the most common metamorphic rocks in the Earth's crust. The 'Acasta Gneiss' in Canada is one of the oldest known rock bodies on Earth, dating back over 4 billion years.
Field identification & locations
Identified by its distinctive color banding (gneissic foliation) and medium-to-coarse grain size. Commonly found in glaciated regions, riverbeds, and mountainous areas where deep crustal rocks have been uplifted.
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