metamorphic
Gneiss
Gneiss
Hardness: 6-7 (Mohs scale); Color: Gray, black, white, and pinkish bands; Luster: Dull to vitreous; Crystal Structure: Foliated; Cleavage: None, but splits along bands; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs scale)
- Color
- Gray, black, white, and pinkish bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Gray, black, white, and pinkish bands; Luster: Dull to vitreous; Crystal Structure: Foliated; Cleavage: None, but splits along bands; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under intense heat and pressure, typically during mountain-building events.
Uses & applications
Used as crushed stone for road construction, as ornamental stone for building facades, floor tiles, and high-end countertops.
Geological facts
Gneiss is some of the oldest rock on Earth, with the Acasta Gneiss in Canada dated to approximately 4 billion years old.
Field identification & locations
Identify it by its characteristic 'gneissic banding'—alternating layers of light-colored minerals (quartz/feldspar) and dark-colored minerals (biotite/amphibole). It is found globally in the cores of mountain ranges.
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