metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Banded grey, white, and black; Luster: Variable, often dull to vitreous; Structure: Foliated with distinct compositional banding (alternating dark and light layers); Cleavage: None (poor splitting along bands).
- Hardness
- 6-7 on Mohs scale
- Color
- Banded grey, white, and black
- Luster
- Variable, often dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded grey, white, and black; Luster: Variable, often dull to vitreous; Structure: Foliated with distinct compositional banding (alternating dark and light layers); Cleavage: None (poor splitting along bands).
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme temperature and pressure. It often dates back hundreds of millions to billions of years.
Uses & applications
Used as a dimension stone for flooring, ornamental facades, gravestones, and crushed stone for road construction and landscaping.
Geological facts
Gneiss contains some of the oldest known rock on Earth, such as the Acasta Gneiss in Canada which is roughly 4 billion years old. The banding is caused by the segregation of light (quartz/feldspar) and dark (mica/amphibole) minerals.
Field identification & locations
Identify in the field by looking for 'gneissose banding' (thick mineral layers that don't easily split) and coarse grain size. Often found in deeply eroded mountain ranges or glacial till.
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