metamorphic
Banded gneiss
Gneiss, commonly quartz–feldspar–mica assemblage
AI-generated identification · may be incorrectHardness about 6–7 Mohs; dark gray to black with pale gray and rusty-orange bands, granular texture, and dull to vitreous luster. The irregular light–dark banding reflects foliated, recrystallized minerals, mainly quartz, feldspar, and biotite or amphibole.
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Physical properties
Hardness about 6–7 Mohs; dark gray to black with pale gray and rusty-orange bands, granular texture, and dull to vitreous luster. The irregular light–dark banding reflects foliated, recrystallized minerals, mainly quartz, feldspar, and biotite or amphibole.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent intense high-grade metamorphism, commonly during Precambrian to Paleozoic mountain-building events. It forms deep in continental crust under high temperature and pressure.
Uses & applications
Used as dimension stone, paving, aggregate, retaining walls, and landscape stone; attractive polished pieces are collected but are generally not jewelry gemstones.
Geological facts
Gneissic banding is compositional rather than ordinary sedimentary layering and can preserve evidence of ancient continental crust. Orange areas may be iron-oxide staining or weathered feldspar-rich zones.
Field identification & locations
Identify it by coarse grains and discontinuous light–dark foliation; unlike granite, the minerals are visibly aligned or banded. Common in ancient shield regions such as Scandinavia, Canada, the northeastern United States, India, and Australia.
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