metamorphic
Granite Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 (Mohs scale); Color: Mottled grey, pink, and black; Luster: Dull to vitreous; Crystal structure: Coarse-grained with foliation; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 (Mohs scale)
- Color
- Mottled grey, pink, and black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Mottled grey, pink, and black; Luster: Dull to vitreous; Crystal structure: Coarse-grained with foliation; Specific gravity: 2.6-2.9.
Formation & geological history
Formed through the high-grade metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks under extreme heat and pressure, often during mountain-building events (orogeny). Age can range from hundreds of millions to billions of years.
Uses & applications
Used primarily in construction as crushed stone or landscaping material; high-quality boulders are used for architectural decorative stones.
Geological facts
Gneiss is one of the oldest rock types found on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. It is defined by its characteristic 'gneissic banding' or segregation of minerals.
Field identification & locations
Identify in the field by looking for alternating bands of light-colored (quartz/feldspar) and dark-colored (biotite/amphibole) minerals. Commonly found in the crystalline cores of mountain ranges.
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