metamorphic
Granitic Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Pink, gray, white, and black; Luster: Vitreous to dull; Crystal structure: Usually medium to coarse-grained with visible banding; Cleavage: Generally poor, breaks along foliation planes; Specific gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Pink, gray, white, and black
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pink, gray, white, and black; Luster: Vitreous to dull; Crystal structure: Usually medium to coarse-grained with visible banding; Cleavage: Generally poor, breaks along foliation planes; Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous precursors like granite. Exposure to intense heat and pressure causes mineral grains to recrystallize and segregate into distinct bands. Often dates back to Precambrian or Paleozoic eras.
Uses & applications
Commonly used in construction as crushed stone for road bases, as dimension stone for architectural facings and flooring, and occasionally used in landscaping and as countertops if polished.
Geological facts
Gneiss is one of the oldest rock types on Earth, with the Acasta Gneiss in Canada dated to about 4 billion years old. The mineral layering is called 'gneissic banding'.
Field identification & locations
Identify in the field by looking for distinct alternating layers of light-colored minerals (quartz/feldspar) and dark minerals (biotite/hornblende). Widely found in continental shield areas and mountain belts.
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