metamorphic
Banded gneiss (likely garnet-biotite or quartz-feldspar gneiss)
Gneiss; predominantly quartz (SiO₂), feldspar, and dark biotite/amphibole
AI-generated identification · may be incorrectHardness about 6–7 Mohs; coarse-grained, nonmetallic, with alternating dark and pale mineral bands and a crystalline texture. The specimen appears naturally weathered or broken, not polished.
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Physical properties
Hardness about 6–7 Mohs; coarse-grained, nonmetallic, with alternating dark and pale mineral bands and a crystalline texture. The specimen appears naturally weathered or broken, not polished.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent high-grade metamorphism, producing mineral segregation and banding during intense heat and pressure. Such rocks commonly formed in Precambrian to Paleozoic continental crust.
Uses & applications
Used as dimension stone, paving, landscaping rock, aggregate, and occasionally as a decorative specimen; generally not suitable for fine jewelry.
Geological facts
Gneissic banding is a metamorphic foliation caused by recrystallization and alignment of minerals, rather than simple sedimentary layering.
Field identification & locations
Identify it by distinct light-dark bands, interlocking crystals, and a lack of vesicles or glassy surfaces; common in ancient mountain belts and exposed continental shield regions worldwide. A precise identification would require a fresh surface or thin-section examination.
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