metamorphic
Gneiss
Gneiss
Hardness: 6-7 (Mohs scale); Color: Banded grey, white, pink, and black; Luster: Vitreous to dull; Crystal structure: Coarse-grained phaneritic; Cleavage: Poorly defined; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 (Mohs scale)
- Color
- Banded grey, white, pink, and black
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Banded grey, white, pink, and black; Luster: Vitreous to dull; Crystal structure: Coarse-grained phaneritic; Cleavage: Poorly defined; Specific gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under high temperature and pressure. Often Precambrian to Paleozoic in age.
Uses & applications
Used as a dimension stone for flooring, ornamental facades, cemetery markers, and crushed stone for road construction and concrete aggregate.
Geological facts
Gneiss is known for its distinct 'gneissic banding,' which is the segregation of light and dark minerals. Some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada (approx. 4 billion years old), are types of gneiss.
Field identification & locations
Identify in the field by looking for distinct foliation or compositional banding. Commonly found in the cores of mountain ranges and eroded shield areas like the Appalachian Mountains or the Canadian Shield.
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