metamorphic
Granite Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Pinkish-orange, black, and grey; Luster: Dull to vitreous; Structure: Foliated with characteristic 'gneissic banding'; Cleavage: Poorly defined overall; Specific Gravity: 2.6 - 2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Pinkish-orange, black, and grey
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pinkish-orange, black, and grey; Luster: Dull to vitreous; Structure: Foliated with characteristic 'gneissic banding'; Cleavage: Poorly defined overall; Specific Gravity: 2.6 - 2.9.
Formation & geological history
Formed through high-grade regional metamorphism of igneous granite or sedimentary protoliths under intense heat and pressure. Usually associated with Precambrian or Paleozoic mountain-building events.
Uses & applications
Used as dimension stone for architecture, countertops, flooring, and as crushed stone for road construction and aggregate.
Geological facts
Gneiss is among the oldest known rocks on Earth, with the Acasta Gneiss in Canada dating back over 4 billion years. The distinct banding occurs because minerals segregate into separate layers during metamorphism.
Field identification & locations
Identify in the field by looking for distinct alternating bands of light-colored minerals (quartz/feldspar) and dark minerals (biotite/amphibole). It does not split easily along planes like schist.
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