metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and black with visible banding; Luster: Dull to vitreous; Crystal structure: Coarse-grained crystalline with distinct foliation; Specific gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, pink, white, and black with visible banding
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and black with visible banding; Luster: Dull to vitreous; Crystal structure: Coarse-grained crystalline with distinct foliation; Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary protoliths under extreme temperature and pressure. Common in Precambrian basement rocks and mountain belts like the Alps.
Uses & applications
Used as building stone, pavers, decorative landscaping, and occasionally as interior countertop material ('granite' in commercial trade).
Geological facts
The banding in gneiss, called gneissic banding, is the result of mineral segregation into layers. Some of the oldest rocks on Earth, such as the Acasta Gneiss (4.03 billion years old), are of this type.
Field identification & locations
Identify by the 'zebra stripe' or banded appearance of alternating light (quartz/feldspar) and dark (biotite/hornblende) minerals. Extremely common in the Southern Alps and Pohorje Mountains of Slovenia (near 46.5°N, 15.6°E).
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