metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray, green, or pinkish with dark bands; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissic banding); Cleavage: Poor; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, green, or pinkish with dark bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, green, or pinkish with dark bands; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissic banding); Cleavage: Poor; Specific Gravity: 2.6-2.9.
Formation & geological history
High-grade regional metamorphism of igneous or sedimentary rocks under high temperature and pressure. Often Precambrian in age, forming deep in the crust during mountain-building events.
Uses & applications
Used as crushed stone for road construction, decorative building stone, countertops, and landscaping.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The banding occurs because minerals segregate into layers during intense heat.
Field identification & locations
Identify by the alternating layers of light-colored (quartz/feldspar) and dark-colored (biotite/amphibole) minerals. Commonly found in core regions of mountain ranges and glacial till.
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