metamorphic
Gneiss
Gneiss
Hardness: 6-7 Mohs; Color: Pinkish-tan with dark grey/black bands; Luster: Dull to vitreous; Crystal structure: Coarse-grained, crystalline; Cleavage: None (distinct foliation instead); Specific gravity: 2.6-2.9
- Hardness
- 6-7 Mohs
- Color
- Pinkish-tan with dark grey/black bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Pinkish-tan with dark grey/black bands; Luster: Dull to vitreous; Crystal structure: Coarse-grained, crystalline; Cleavage: None (distinct foliation instead); Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing granitic or sedimentary rocks under high temperature and pressure. Often Precambrian in age.
Uses & applications
Used as building stone, architectural facing, paving, and crushed stone for road construction and landscaping.
Geological facts
Gneiss is one of the oldest rock types on Earth, with the Acasta Gneiss in Canada estimated to be over 4 billion years old. The term comes from an Old High German verb meaning 'to spark'.
Field identification & locations
Identify by 'gneissose banding'—distinct layers of light minerals (quartz, feldspar) and dark minerals (biotite, hornblende). Common in cratonic shields and mountain cores.
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