metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 (Mohs scale); Color: Gray, brown, or pink with dark and light mineral bands; Luster: Dull to vitreous; Crystal structure: Coarse-grained foliated; Cleavage: None (schistosity is absent)
- Hardness
- 6-7 (Mohs scale)
- Color
- Gray, brown, or pink with dark and light mineral bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Gray, brown, or pink with dark and light mineral bands; Luster: Dull to vitreous; Crystal structure: Coarse-grained foliated; Cleavage: None (schistosity is absent)
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under high temperature and pressure. Often Precambrian in age.
Uses & applications
Used as crushed stone for road construction, building stone for cladding and flooring, and occasionally as ornamental pieces.
Geological facts
Gneiss is one of the oldest rock types found on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old.
Field identification & locations
Identify by 'gneissose banding' (distinct light and dark layers). Found in the roots of mountain ranges and ancient continental shields. Look for alternating bands of quartz/feldspar and biotite/hornblende.
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