metamorphic
Gneiss
High-grade regional metamorphic rock
Hardness: 6-7 on Mohs scale; Color: Dark grey, black, and white; Luster: Pearly to dull; Crystal structure: Coarse-grained with distinct foliation or banding; Cleavage: Generally poor, breaks along bands.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey, black, and white
- Luster
- Pearly to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey, black, and white; Luster: Pearly to dull; Crystal structure: Coarse-grained with distinct foliation or banding; Cleavage: Generally poor, breaks along bands.
Formation & geological history
Forms under high temperature and pressure conditions (regional metamorphism) from pre-existing igneous or sedimentary rocks like granite or shale. Often associated with mountain-building events.
Uses & applications
Used as crushed stone for road construction, as dimension stone for building facades, countertops, and landscaping.
Geological facts
Gneiss is one of the oldest known rock types on Earth, with the Acasta Gneiss in Canada being dated to approximately 4 billion years old.
Field identification & locations
Identify by the 'gneissic banding'—alternating layers of light (quartz/feldspar) and dark (biotite/amphibole) minerals. Found in the roots of mountain ranges and stable continental interiors.
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