metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray, black, and white bands; Luster: Dull to pearly; Crystal structure: Crystalline with high-grade foliation; Cleavage: Poor, usually breaks along mineral boundaries.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, black, and white bands
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, black, and white bands; Luster: Dull to pearly; Crystal structure: Crystalline with high-grade foliation; Cleavage: Poor, usually breaks along mineral boundaries.
Formation & geological history
Formed under high-temperature and high-pressure conditions deep within the earth's crust, typically from igneous or sedimentary protoliths such as granite or shale during regional metamorphism.
Uses & applications
Used as a crushed stone in road construction, as dimension stone for building facades, floor tiles, and grave markers; sometimes used as landscape stone or decorative aggregates.
Geological facts
Gneiss is among the oldest known rocks on Earth, with the Acasta Gneiss in Canada dating back approximately 4 billion years. Its name is derived from a Middle High German verb meaning 'to spark' or 'glimmer'.
Field identification & locations
Identified in the field by its distinct alternating layers of light-colored (felsic) and dark-colored (mafic) minerals, known as gneissic banding. Found globally in Precambrian shields and mountain ranges.
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