metamorphic
Gneiss
Gneiss
Foliated with distinct light and dark mineral bands (gneissic banding). Hardness: 6-7 on Mohs scale. Luster: Dull to vitreous. Grain size: Coarse. Color: Gray, white, pink, and black. Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, white, pink, and black
- Luster
- Dull to vitreous
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Physical properties
Foliated with distinct light and dark mineral bands (gneissic banding). Hardness: 6-7 on Mohs scale. Luster: Dull to vitreous. Grain size: Coarse. Color: Gray, white, pink, and black. Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary (shale/mudstone) protoliths under extreme heat and pressure. Commonly found in the cores of mountain ranges and ancient continental shields.
Uses & applications
Used as a dimension stone for flooring, cemetery markers, vanities, and building veneers. Lower grade stones are crushed for road construction and railroad ballast.
Geological facts
Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The term 'gneiss' comes from a Middle High German verb meaning to spark or glisten.
Field identification & locations
Identify by looking for 'pinstriping' or distinct alternating bands of minerals like quartz/feldspar (light) and biotite/hornblende (dark). It typically shows no cleavage and is extremely durable. Found widely in New England, the Appalachians, and the Canadian Shield.
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