metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: typically banded gray, white, and dark minerals; Luster: dull to vitreous; Crystal structure: foliated/gneissose banding; Cleavage: poor; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- typically banded gray, white, and dark minerals
- Luster
- dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically banded gray, white, and dark minerals; Luster: dull to vitreous; Crystal structure: foliated/gneissose banding; Cleavage: poor; Specific gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary (shale/sandstone) rocks under extreme pressure and temperature. Foundations of continents, often billions of years old.
Uses & applications
Used as a dimension stone for flooring, cemetery markers, facing on buildings, and as crushed stone for road construction and landscaping.
Geological facts
The Acasta Gneiss in Canada is one of the oldest known intact crustal rock fragments on Earth, dated to approximately 4.03 billion years ago.
Field identification & locations
Identify by distinct 'gneissose banding' – alternating layers of light-colored (quartz/feldspar) and dark-colored (biotite/amphibole) minerals. Found in mountainous regions and glacial tills.
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