metamorphic
Gneiss
Metamorphic rock (Gneiss)
Hardness: 6-7 on the Mohs scale; Color: Gray, white, pink, or multi-colored with alternating light and dark bands (gneissic banding); Luster: Dull to sub-vitreous; Crystal structure: Granular, foliated texture with distinct mineral segregation.
- Hardness
- 6-7 on the Mohs scale
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on the Mohs scale; Color: Gray, white, pink, or multi-colored with alternating light and dark bands (gneissic banding); Luster: Dull to sub-vitreous; Crystal structure: Granular, foliated texture with distinct mineral segregation.
Formation & geological history
Formed by high-grade regional metamorphism, typically from preexisting igneous rocks (like granite) or sedimentary rocks (like shale) under extreme heat and pressure deep within the Earth's crust, often dating back billions of years to the Precambrian.
Uses & applications
Used primarily in construction as crushed stone for road bases, building facades, landscaping, and occasionally as dimension stone for countertops or monuments.
Geological facts
Gneiss contains some of the oldest known rocks on Earth, with certain formations dating back over 4 billion years. The distinct alternating light and dark bands are known as gneissic banding.
Field identification & locations
Identified in the field by its prominent alternating light-colored bands of quartz and feldspar and dark-colored bands of biotite or amphibole. Commonly found in ancient continental shields and mountain cores worldwide.
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