metamorphic
Gneiss (Granite Gneiss)
Gneiss
Hardness: 6-7 Mohs; Color: Salt and pepper appearance with pink, white, and black bands; Luster: Dull to vitreous; Crystal Structure: Foliated with coarse mineral grains; Cleavage: Poor, but splits along foliation planes; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 Mohs
- Color
- Salt and pepper appearance with pink, white, and black bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Salt and pepper appearance with pink, white, and black bands; Luster: Dull to vitreous; Crystal Structure: Foliated with coarse mineral grains; Cleavage: Poor, but splits along foliation planes; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed from high-grade regional metamorphism of granite or sedimentary rocks like shale under intense heat and pressure deep within the Earth's crust. Most specimens range from hundreds of millions to billions of years old.
Uses & applications
Used in construction as crushed stone, dimension stone for flooring, cemetery markers, architectural cladding, and occasionally as landscaping decor.
Geological facts
Gneiss is one of the oldest known rock types on Earth, with the Acasta Gneiss in Canada being dated at nearly 4 billion years old. The term comes from the Middle High German word 'gneist', meaning to spark.
Field identification & locations
Identify by 'gneissoid banding' where layers of different minerals (like mica, quartz, and feldspar) separate into alternating light and dark stripes. Found commonly in mountain cores and ancient continental shields.
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