metamorphic
Gneis
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: alternating light (quartz/feldspar) and dark (biotite/hornblende) bands; Luster: vitreous to dull; Crystal structure: granular, foliated; Cleavage: poor; Specific gravity: 2.6-3.0.
- Hardness
- 6-7 on Mohs scale
- Luster
- vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: alternating light (quartz/feldspar) and dark (biotite/hornblende) bands; Luster: vitreous to dull; Crystal structure: granular, foliated; Cleavage: poor; Specific gravity: 2.6-3.0.
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary (shale/mudstone) protoliths under intense heat and pressure deep within the Earth's crust, often during mountain-building events.
Uses & applications
Used as a dimension stone for architectural facings, flooring, and curbing. Also used as crushed stone for road construction and landscaping.
Geological facts
The Acasta Gneiss in northern Canada is one of the oldest known crustal rocks on Earth, dating back approximately 4.03 billion years. Its name is derived from the German word 'Gneist' meaning 'spark'.
Field identification & locations
Identified by 'gneissic banding'—the segregation of minerals into alternating layers of different colors. It is common in the deeply eroded cores of mountain ranges and in ancient continental shields.
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