metamorphic
Gneiss (Gneissic Granite)
Gneiss
Hardness: 6-7 on Mohs scale; Color: Pink, gray, and black bands; Luster: Vitreous to dull; Crystal structure: Course-grained, foliation (banding); Cleavage: Distinct in mica layers; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Pink, gray, and black bands
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pink, gray, and black bands; Luster: Vitreous to dull; Crystal structure: Course-grained, foliation (banding); Cleavage: Distinct in mica layers; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary rocks (likely granite or shale) under extreme pressure and temperature. These rocks are often millions to billions of years old.
Uses & applications
Used as building stone, decorative landscaping, flooring, and gravestones. It is also crushed for use in road construction and concrete aggregate.
Geological facts
Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The term comes from the German word 'Gneist', meaning 'spark'.
Field identification & locations
Identify by 'gneissic banding'—distinct layered appearances of different colors. Commonly found in the cores of mountain ranges and glacial deposits. For collectors, look for alternating bands of pink feldspar and dark biotite.
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