metamorphic
Granite Gneiss
Gneiss (quartz-feldspar dominated)
Hardness: 6-7 Mohs; Color: Pink, gray, and black banding; Luster: Vitreous to dull; Crystal structure: Course-grained, crystalline; Cleavage: Generally poor, breaks along foliation; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 Mohs
- Color
- Pink, gray, and black banding
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 Mohs; Color: Pink, gray, and black banding; Luster: Vitreous to dull; Crystal structure: Course-grained, crystalline; Cleavage: Generally poor, breaks along foliation; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of igneous rock (granite) or sedimentary rocks. The minerals recrystallize under intense heat and pressure, often at deep crustal levels during mountain-building events.
Uses & applications
Used as a high-quality building stone, crushed aggregate for road construction, ornamental stone, and in some cases, for kitchen countertops where it is often sold as 'granite'.
Geological facts
Gneiss is some of the oldest rock on Earth, with the Acasta Gneiss in Canada being dated to approximately 4 billion years old. The distinctive banding is called 'gneissose banding'.
Field identification & locations
Identify in the field by looking for alternating light-colored (felsic) and dark-colored (mafic) bands. It is commonly found in the roots of mountain ranges and in ancient continental shields (like the Canadian Shield).
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