metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Gray, pink, or brownish bands; Luster: Pearly to dull; Crystal structure: Coarse-grained foliated; Cleavage: Poorly defined; Specific gravity: 2.6-3.0
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, pink, or brownish bands
- Luster
- Pearly to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, pink, or brownish bands; Luster: Pearly to dull; Crystal structure: Coarse-grained foliated; Cleavage: Poorly defined; Specific gravity: 2.6-3.0
Formation & geological history
Formed by the high-pressure, high-temperature metamorphism of granite or sedimentary rocks like shale. Original minerals recrystallize into distinct bands through a process called gneissic banding.
Uses & applications
Commonly used in construction as crushed stone for road bases, as a building stone for walls and floors, and in landscaping.
Geological facts
Gneiss is one of the oldest known rock types on Earth, with some specimens in Canada dated to over 4 billion years old. The banding occurs because minerals sort themselves by density during extreme pressure.
Field identification & locations
Identify by looking for 'gneissic banding,' which is alternating layers of light-colored (quartz/feldspar) and dark-colored (biotite/amphibole) minerals. Found in ancient mountain belts and glacial deposits.
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