metamorphic
Granite Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and black; Luster: Dull to vitreous; Structure: Foliated with distinct banding; Cleavage: Poor, typically breaks along grain boundaries.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, pink, white, and black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and black; Luster: Dull to vitreous; Structure: Foliated with distinct banding; Cleavage: Poor, typically breaks along grain boundaries.
Formation & geological history
Formed through high-grade regional metamorphism of granite or sedimentary protoliths. Exposed to intense heat and pressure, causing minerals to recrystallize and segregate into bands. Often found in Precambrian shield areas.
Uses & applications
Used primarily as crushed stone for road construction and aggregate. High-quality slabs are used for architectural facing, monuments, and occasionally countertops if polished.
Geological facts
Gneiss is some of the oldest rock found on Earth, with the Acasta Gneiss in Canada dating back over 4 billion years. The name comes from a German word meaning 'to spark' or 'glimmer'.
Field identification & locations
Identify by looking for 'gneissose banding' (alternating layers of light and dark minerals). It is commonly found in mountain cores and glacial deposits across North America and Europe.
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