metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Grey, black, white in banded layers; Luster: Dull to pearly; Crystal structure: Coarse-grained foliated; Cleavage: Poor, but splits along bands; Specific gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Grey, black, white in banded layers
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Grey, black, white in banded layers; Luster: Dull to pearly; 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 igneous or sedimentary rocks under extreme pressure and temperature. Distinctive banding (gneissic foliation) occurs as minerals segregate during deformation.
Uses & applications
Commonly used in construction as crushed stone for road bases, as a building stone for facades, and occasionally for counter-tops if aesthetically pleasing.
Geological facts
Gneiss contains some of the oldest known rock units on Earth, such as the Acasta Gneiss in Canada, which is roughly 4.03 billion years old.
Field identification & locations
Identify in the field by looking for 'gneissic banding'—alternating light and dark mineral layers. Found in the cores of mountain ranges and ancient continental shields like the Canadian Shield or the Appalachians.
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