metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 (Mohs); Color: Alternating dark (biotite, hornblende) and light (quartz, feldspar) bands; Luster: Vitreous to dull; Structure: Gneissose banding/foliation; Cleavage: Generally none, though mineral components may have individual cleavage.
- Hardness
- 6-7 (Mohs)
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs); Color: Alternating dark (biotite, hornblende) and light (quartz, feldspar) bands; Luster: Vitreous to dull; Structure: Gneissose banding/foliation; Cleavage: Generally none, though mineral components may have individual cleavage.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks at extreme temperatures (above 600°C) and pressures. Often associated with mountain-building events (orogenies).
Uses & applications
Used primarily as crushed stone for road construction, concrete aggregate, and as a decorative building stone (veneers, tiles, and countertops once polished).
Geological facts
Gneiss is among the oldest known rocks on Earth. The Acasta Gneiss in Canada is dated at approximately 4.03 billion years old, nearly as old as the planet itself.
Field identification & locations
Identify in the field by looking for distinct 'gneissose banding'—alternating layers of different minerals. Unlike schist, it does not split easily along planes. Often found in the cores of mountain ranges and stable continental shields.
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