metamorphic
Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Gray, black, pink, or white bands; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissic banding); Cleavage: Poor, usually breaks along uneven surfaces.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, black, pink, or white bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, black, pink, or white bands; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissic banding); Cleavage: Poor, usually breaks along uneven surfaces.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under extreme pressure and high temperature. This specimen shows typical segregation of light and dark minerals into bands.
Uses & applications
Used primarily as a construction material (crushed stone for road bases), dimension stone for tiles and countertops, and occasionally for landscaping.
Geological facts
Gneiss contains some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4 billion years old.
Field identification & locations
Identify in the field by looking for 'gneissic banding' where light-colored minerals (quartz/feldspar) and dark-colored minerals (biotite/amphibole) are separated into distinct layers. Often found in mountainous regions or glacial deposits.
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