metamorphic
Gneiss
High-grade regional metamorphic rock (Foliated)
Hardness: 6-7 (Mohs scale); Color: Gray, black, white, pink; Luster: Pearly to dull; Structure: Gneissic banding with distinct alternating light/dark mineral layers; Specific gravity: 2.7-3.2
- Hardness
- 6-7 (Mohs scale)
- Color
- Gray, black, white, pink
- Luster
- Pearly to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Gray, black, white, pink; Luster: Pearly to dull; Structure: Gneissic banding with distinct alternating light/dark mineral layers; Specific gravity: 2.7-3.2
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary protoliths under extreme temperature (600-700°C) and pressure. Found in Precambrian shields or mountain cores.
Uses & applications
Used in construction as crushed stone or fill, decorative flooring, countertops, and facing stones for buildings due to its durability and aesthetic banding.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old and is considered the oldest known crustal rock.
Field identification & locations
Identify by the characteristic 'gneissic banding' or foliation which looks like stripes but does not cleave easily like schist. Common in metamorphic belts and mountain ranges.
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