metamorphic
Granite Gneiss
Metamorphic Rock (Quartz, Potassium Feldspar, Plagioclase, Biotite/Amphibole)
Hardness 6-7 on Mohs scale. Mottled pink, white, grey, and dark green/black banding. Vitreous to dull luster depending on surface weathering. Coarse-grained crystalline texture with visible mineral segregation (foliation).
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Physical properties
Hardness 6-7 on Mohs scale. Mottled pink, white, grey, and dark green/black banding. Vitreous to dull luster depending on surface weathering. Coarse-grained crystalline texture with visible mineral segregation (foliation).
Formation & geological history
Formed under intense regional metamorphism with high temperature and pressure, recrystallizing pre-existing igneous rocks like granite or sedimentary rocks like sandstone. Frequently smoothed by glacial or stream transport into river pebbles.
Uses & applications
Used widely as crushed stone in construction, decorative landscaping, dimension stone, counter tops, and as educational geological samples.
Geological facts
Gneiss is one of the oldest rock types found on Earth, with the Acasta Gneiss in Canada dating back nearly 4 billion years. The distinct alternating light and dark mineral bands are caused by directional pressure deep within the Earth's crust.
Field identification & locations
Identified by alternating light bands of quartz and pink orthoclase feldspar alongside dark bands of biotite mica or hornblende. Commonly found along stream beds, glacial drift regions, and mountain belts worldwide.
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