metamorphic
Gneiss
High-grade Metamorphic Gneiss
Hardness: 6-7 on Mohs scale. Color: Gray, black, pink, or white alternating bands. Luster: Dull to vitreous. Crystal structure: Faliated, medium to coarse-grained. Cleavage: Poor, usually breaks along mineral boundaries. Specific gravity: 2.6-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, black, pink, or white alternating bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Gray, black, pink, or white alternating bands. Luster: Dull to vitreous. Crystal structure: Faliated, medium to coarse-grained. Cleavage: Poor, usually breaks along mineral boundaries. Specific gravity: 2.6-3.0.
Formation & geological history
Forms through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths like granite or shale). It is subjected to intense heat and pressure deep within the Earth's crust, often during mountain-building events (orogenies).
Uses & applications
Widely used as crushed stone in road construction and as ballast. In polished form, it is used for building facades, floor tiles, and grave markers due to its durability and aesthetic banding.
Geological facts
Gneiss is some of the oldest rock on Earth, with the Acasta Gneiss in Canada dated to approximately 4 billion years old. The term 'gneiss' comes from an old German verb meaning 'to spark' or 'to glisten'.
Field identification & locations
Identified in the field by its distinct 'gneissic banding'—alternating layers of light (felsic) and dark (mafic) minerals. Common in the cores of mountain ranges and ancient continental shields. Does not split easily along planes like schist.
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