metamorphic
Gneiss
Metamorphic Gneiss
Hardness 6-7 on Mohs scale. Luster is earthy to vitreous. Color ranges from light gray, white, to tan with distinctive dark biotite or amphibole banding. Non-porous texture with medium to coarse grains.
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Physical properties
Hardness 6-7 on Mohs scale. Luster is earthy to vitreous. Color ranges from light gray, white, to tan with distinctive dark biotite or amphibole banding. Non-porous texture with medium to coarse grains.
Formation & geological history
Formed by high-grade regional metamorphism of igneous (like granite) or sedimentary rocks under extreme heat and pressure, leading to segregation of light and dark minerals into alternating bands.
Uses & applications
Used as a crushed stone in construction, dimension stone for buildings, landscaping, countertops, and as educational or decorative rock specimens.
Geological facts
The term 'gneiss' comes from an old German mining term meaning spark or bright, referencing its shiny mineral grains. The distinct layered look is known as 'gneissic banding'.
Field identification & locations
Identified in the field by alternating light quartz/feldspar bands and dark mica/amphibole bands. Commonly found in ancient shield areas and mountain cores worldwide like the Appalachian or Rocky Mountains.
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