metamorphic
Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Gray with dark bands/streaks; Luster: Dull to sub-vitreous; Structure: Foliated with distinct banding (gneissose banding); Cleavage: Poor, usually breaks along irregular surfaces.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray with dark bands/streaks
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray with dark bands/streaks; Luster: Dull to sub-vitreous; Structure: Foliated with distinct banding (gneissose banding); Cleavage: Poor, usually breaks along irregular surfaces.
Formation & geological history
Formed through high-pressure, high-temperature regional metamorphism of igneous (like granite) or sedimentary (like shale) protoliths. Often dates back to the Precambrian era and occurs in the deep crust during mountain-building events.
Uses & applications
Commonly used as crushed stone for road construction, as dimension stone for building facades, floor tiles, and as decorative landscaping stones or garden ornaments.
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 'to glisten'.
Field identification & locations
Identified by its characteristic alternating light and dark bands (foliation). Look for coarse grains and a lack of easy cleavage compared to schist. Found in the cores of mountain ranges and ancient continental shields.
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