metamorphic
Granitic Gneiss
Gneiss (Metamorphic rock of granitic composition)
Hardness: 6-7 on Mohs scale. Color: Tan, white, and gray with dark bands. Luster: Dull to vitreous. Crystal structure: Foliated (banded), phaneritic texture with coarse grains of quartz, feldspar, and mica.
- Hardness
- 6-7 on Mohs scale
- Color
- Tan, white, and gray with dark bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Tan, white, and gray with dark bands. Luster: Dull to vitreous. Crystal structure: Foliated (banded), phaneritic texture with coarse grains of quartz, feldspar, and mica.
Formation & geological history
Formed through high-grade regional metamorphism of igneous granite or sedimentary protoliths. The rock has been subjected to extreme heat and pressure, causing minerals to segregate into distinct bands through recrystallization.
Uses & applications
Used in construction as crushed stone for road bases, as a building stone for monuments or facades, and as dimension stone for countertops. Lower value specimens are used for landscaping.
Geological facts
Gneiss is some of the oldest rock on Earth, with the Acasta Gneiss in Canada dating back over 4 billion years. It is defined by its banding pattern, a process known as gneissic banding.
Field identification & locations
Identify by looking for distinct layers or bands of color (foliation) and large visible grains. Common in mountainous regions or stable continental shields like the Canadian Shield or the Appalachian Mountains.
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