metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray, black, and tan banding; Luster: Dull to pearly; Crystal structure: Foliated (banded); Cleavage: Poor; Specific gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, black, and tan banding
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, black, and tan banding; Luster: Dull to pearly; Crystal structure: Foliated (banded); Cleavage: Poor; Specific gravity: 2.6-2.9
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense heat and pressure, causing minerals to segregate into distinct bands. Often Precambrian in age.
Uses & applications
Used as crushed stone for road construction, building stone for flooring and gravestones, and as an ornamental landscaping rock.
Geological facts
Gneiss is one of the oldest rock types on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The name comes from the Middle High German word 'gneist', meaning spark.
Field identification & locations
Identified by 'gneissic banding' or alternating layers of light and dark minerals. Common in mountainous regions and glacial drift. Collectors look for unique swirl patterns.
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