metamorphic
Gneiss
High-grade metamorphic rock (Gneiss)
Hardness: 6-7 Mohs; Color: Banded grey, tan, and brown with streaks of pink; Luster: Dull to sub-vitreous; Crystal structure: Crystalline with foliation (gneissic banding); Cleavage: Poor; Specific gravity: 2.6-2.9
- Hardness
- 6-7 Mohs
- Color
- Banded grey, tan, and brown with streaks of pink
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Banded grey, tan, and brown with streaks of pink; Luster: Dull to sub-vitreous; Crystal structure: Crystalline with foliation (gneissic banding); Cleavage: Poor; Specific gravity: 2.6-2.9
Formation & geological history
Formed via regional metamorphism of pre-existing igneous (granite) or sedimentary rocks under extreme pressure and high temperature. Often associated with mountain-building events (orogeny) dating back millions to billions of years.
Uses & applications
Commonly used as crushed stone in construction (roads), as dimension stone for architectural facades, countertops, and occasionally as decorative landscaping stones.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4.03 billion years old. It is distinguished by its distinct light and dark colored mineral bands.
Field identification & locations
Identify in the field by looking for distinct 'banding' or layering of minerals like quartz, feldspar, and biotite. Found in core areas of continents and mountain ranges. Collectors look for high contrast in the bands or presence of garnets.
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