metamorphic
Banded gneiss (likely migmatitic gneiss)
Gneiss; quartz–feldspar–mica metamorphic rock, locally approaching migmatite
AI-generated identification · may be incorrectTypically Mohs 6–7; gray, tan, and dark brown with coarse, irregular foliation and swirled banding; vitreous quartz, feldspar-rich areas, and dull-to-silky mica. No obvious cleavage is visible at specimen scale; density is commonly about 2.6–2.8 g/cm³.
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Physical properties
Typically Mohs 6–7; gray, tan, and dark brown with coarse, irregular foliation and swirled banding; vitreous quartz, feldspar-rich areas, and dull-to-silky mica. No obvious cleavage is visible at specimen scale; density is commonly about 2.6–2.8 g/cm³.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent high-grade metamorphism, commonly at roughly 600–800°C, causing mineral recrystallization and compositional banding; many examples are Precambrian (over 541 million years old).
Uses & applications
Used as dimension stone, decorative landscaping rock, aggregate, and occasionally polished architectural stone; attractive pieces are collected but are generally not jewelry material.
Geological facts
The contorted light-and-dark layers reflect intense ductile deformation and mineral segregation; partial melting can produce pale veins and patches, giving migmatitic textures.
Field identification & locations
Identify it by alternating light quartz/feldspar bands and dark biotite- or amphibole-rich layers, unlike uniform granite; common in ancient continental shields and mountain belts, including Canada, Scandinavia, Scotland, India, and the northeastern United States.
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