metamorphic
Banded Gneiss
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 (Mohs scale), Color: Alternating bands of dark (often biotite, hornblende) and light (quartz, feldspar/orange-pink orthoclase), Luster: Dull to vitreous, Structure: Gneissic foliation/banding, Cleavage: Poorly defined overall.
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Alternating bands of dark (often biotite, hornblende) and light (quartz, feldspar/orange-pink orthoclase), Luster: Dull to vitreous, Structure: Gneissic foliation/banding, Cleavage: Poorly defined overall.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense heat and pressure. The distinct bands result from chemical segregation (metamorphic differentiation) deep within the crust.
Uses & applications
Used in construction as crushed stone, dimension stone for countertops and floors, and in landscaping. Polished river stones like this are valued by collectors for aesthetic decor.
Geological facts
Gneiss contains some of the oldest rock formations on Earth, such as the Acasta Gneiss in Canada, which is over 4 billion years old. The banding implies the rock did not melt, but minerals rearranged while solid.
Field identification & locations
Identify by distinct 'zebra-like' stripes or bands of contrasting mineral colors. Often found in riverbeds within mountainous regions where glacial or water erosion has smooth the surface.
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