metamorphic
Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 (Mohs scale), Color: Alternating light and dark bands (foliation), Luster: Dull to pearly, Crystal structure: Coarse-grained, non-distinct crystals, Cleavage: Poor, Specific gravity: 2.6-2.9
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Alternating light and dark bands (foliation), Luster: Dull to pearly, Crystal structure: Coarse-grained, non-distinct crystals, Cleavage: Poor, Specific gravity: 2.6-2.9
Formation & geological history
Forms from the high-temperature and high-pressure metamorphism of granite or sedimentary rocks like shale; typically Precambrian or Paleozoic in age
Uses & applications
Used as building stone, decorative landscaping, flooring, and crushed stone for road construction
Geological facts
The Gneiss in the Acasta Gneiss complex in Canada is among the oldest known intact crustal rocks on Earth, dated to approximately 4.03 billion years
Field identification & locations
Identify by 'gneissose banding' or segregated layers of dark (biotite/hornblende) and light (quartz/feldspar) minerals. Found in mountainous regions and continental shields
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