metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Banded gray, white, and pink; Luster: Dull to vitreous; Structure: Foliated with distinct bands; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Banded gray, white, and pink
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded gray, white, and pink; Luster: Dull to vitreous; Structure: Foliated with distinct bands; Specific Gravity: 2.6-2.9
Formation & geological history
Formed by high-grade regional metamorphism of igneous or sedimentary rocks under extreme heat and pressure, typically in the deep crust during mountain-building events.
Uses & applications
Used as crushed stone for road construction and railroad ballast; high-quality varieties are used as dimensional stone for counter tops and building facades.
Geological facts
Gneiss is one of the oldest rock types on Earth, with the Acasta Gneiss in Canada dated to approximately 4 billion years old.
Field identification & locations
Identify in the field by looking for 'gneissose banding'—alternating light and dark layers. Common in continental shields and mountain ranges like the Appalachians or Alps.
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