metamorphic
Granite Gneiss
High-grade regional metamorphic rock (Quartz-Feldspar Gneiss)
Hardness: 6-7 on Mohs scale; Color: Pinkish-red, grey, and dark brown; Luster: Dull to vitreous; Crystal structure: Crystalline with foliation (gneissic banding); Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Pinkish-red, grey, and dark brown
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pinkish-red, grey, and dark brown; Luster: Dull to vitreous; Crystal structure: Crystalline with foliation (gneissic banding); Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous granitic precursors or sedimentary protoliths. It usually forms deep within the Earth's crust under intense pressure and temperature, often during mountain-building events (orogenies).
Uses & applications
Used primarily in heavy construction, as road aggregate, architectural stone, and decorative landscaping. High-quality slabs are used for countertops.
Geological facts
Gneiss is among the oldest rocks on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The distinct banding is caused by the segregation of light (felsic) and dark (mafic) minerals.
Field identification & locations
Identify in the field by looking for 'gneissic banding'—alternating layers of light and dark minerals. It is very hard and will not scratch easily with a steel blade. Found in the roots of old mountain ranges and continental shields.
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