metamorphic
Gneiss
Gneiss
Hardness: 6-7 (Mohs); Color: Banded grey, white, and black with visible mica; Luster: Dull to vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poor, but splits along bands; Specific gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs)
- Color
- Banded grey, white, and black with visible mica
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs); Color: Banded grey, white, and black with visible mica; Luster: Dull to vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poor, but splits along bands; Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme heat and pressure, typically found in ancient mountain belts and continental shields.
Uses & applications
Used in construction as crushed stone for road bases and as a decorative building stone (dimension stone) for flooring, gravestones, and countertops.
Geological facts
The oldest known rocks on Earth are the Acasta Gneiss in Canada, which are over 4 billion years old. The distinctive banding is called 'gneissic banding' and results from the segregation of different mineral types.
Field identification & locations
Identify in the field by looking for distinct alternating layers of light-colored (quartz/feldspar) and dark-colored (mafic) minerals. Common in the Appalachian Mountains and the Canadian Shield.
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