metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 Mohs; Color: Banded pattern of cream/tan (feldspar/quartz) and black/dark green (biotite/amphibole); Luster: Dull to sub-vitreous; Texture: Coarse-grained with distinct foliation (layering).
- Hardness
- 6-7 Mohs
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Banded pattern of cream/tan (feldspar/quartz) and black/dark green (biotite/amphibole); Luster: Dull to sub-vitreous; Texture: Coarse-grained with distinct foliation (layering).
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale). Subjected to high temperatures and pressures deep within the Earth's crust, often during mountain-building events (orogeny).
Uses & applications
Used as dimension stone for flooring, gravestones, and countertops; also used as crushed stone for road construction and landscaping decorative gravel.
Geological facts
Gneiss is some of the oldest rock found on Earth, with the Acasta Gneiss in Canada dating back approximately 4 billion years. The term 'gneiss' comes from an Old High German verb meaning 'to spark' or 'to glisten'.
Field identification & locations
Identify in the field by its characteristic 'gneissic banding'—alternating light and dark mineral stripes. It is found in the cores of mountain ranges and eroded crystalline shields like the Canadian Shield or the Appalachians.
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