metamorphic
Granitic Gneiss
Felsic Polycrystalline Gneiss
Hardness: 6-7 (Mohs scale), Color: Pink, white, and black/dark green bands. Luster: Vitreous to dull. Crystal structure: Macro-crystalline. Cleavage: Poor, but splits along foliation. Specific Gravity: 2.6-2.9.
- Color
- Pink, white, and black/dark green bands
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Pink, white, and black/dark green bands. Luster: Vitreous to dull. Crystal structure: Macro-crystalline. Cleavage: Poor, but splits along foliation. Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through the high-grade metamorphism of granite or sedimentary protoliths under intense heat and pressure, often during mountain-building events (orogeny). Contains prominent veins of pink orthoclase feldspar and white quartz.
Uses & applications
Used primarily in construction as crushed stone or 'granite' countertops and flooring. Occasionally used for decorative landscaping and as educational specimens for geology students.
Geological facts
Gneiss is some of the oldest rock on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The 'zebra' appearance is called gneissic banding.
Field identification & locations
Identify by looking for distinct layers (foliation) and large crystals of feldspar and quartz. Commonly found in the crystalline basement of continental crust and mountain ranges like the Appalachians or Rockies.
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