metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray, white, pink, and black banded; Luster: Dull to vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poorly defined; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, white, pink, and black banded
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, white, pink, and black banded; Luster: Dull to vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poorly defined; Specific gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of igneous or sedimentary rocks at high temperatures and pressures, causing minerals to segregate into distinct bands.
Uses & applications
Used in construction as crushed stone, dimension stone for buildings, flooring, and landscaping. Occasionally used for decorative items when brightly colored.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. It often shows a characteristic 'salt and pepper' appearance.
Field identification & locations
Identified in the field by its distinct alternating light and dark mineral bands (gneissic banding). Commonly found in mountain cores and ancient shield regions. Collectors look for sharp, high-contrast banding.
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