metamorphic
Feldspathic Gneiss (Granite Gneiss)
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Pink, reddish-brown, and grey bands; Luster: Vitreous to dull; Crystal structure: Foliated/Banded; Cleavage: Poor; Specific gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Pink, reddish-brown, and grey bands
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pink, reddish-brown, and grey bands; Luster: Vitreous to dull; Crystal structure: Foliated/Banded; Cleavage: Poor; Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous granite or sedimentary protoliths. It underwent intense heat and pressure, causing minerals to recrystallize and segregate into bands.
Uses & applications
Used as dimension stone in construction (flooring, facades), crushed stone for road aggregate, and as decorative landscape landscaping or tumbling material.
Geological facts
Gneiss is among the oldest known rocks on Earth, with the Acasta Gneiss in Canada dating back over 4 billion years. The banding is caused by the separation of light minerals like quartz/feldspar and dark minerals like biotite.
Field identification & locations
Identify in the field by looking for distinct 'gneissic banding' or foliation layers. It is ubiquitous in continental shields and mountain belts like the Appalachian or Himalayan ranges.
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