metamorphic
Gneiss
Gneiss
Hardness: 6-7 (Mohs scale); Color: Banded grey, black, white, and tan; Luster: Dull to sub-vitreous; Texture: Coarse-grained with distinct foliation (gneissic banding); Cleavage: Poor.
- Hardness
- 6-7 (Mohs scale)
- Color
- Banded grey, black, white, and tan
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Banded grey, black, white, and tan; Luster: Dull to sub-vitreous; Texture: Coarse-grained with distinct foliation (gneissic banding); Cleavage: Poor.
Formation & geological history
Formed from the high-grade metamorphism of igneous or sedimentary rocks (such as granite or shale) under extreme pressure and temperature conditions deep within the Earth's crust. Most specimens are hundreds of millions to billions of years old.
Uses & applications
Used as a decorative stone in architecture, countertops, flooring, and as crushed stone for road construction and landscaping.
Geological facts
The banding in gneiss is caused by the segregation of light minerals like quartz and feldspar from dark minerals like biotite and hornblende during intense metamorphic heating. It is among the oldest known rocks on Earth.
Field identification & locations
Identified by the presence of 'gneissic banding' where minerals are sorted into alternating light and dark layers. Common in mountainous regions and glacial till. Collectors value uniquely folded or colorful patterns.
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