metamorphic
Banded gneiss
Gneiss; chiefly quartz, feldspar, and mica
AI-generated identification · may be incorrectHardness about 6–7 Mohs; coarse-grained with alternating pale quartz–feldspar and darker mica-rich bands, showing a dull to slightly vitreous luster and weak foliation. No obvious cleavage as a whole rock, though mica layers cleave readily.
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Physical properties
Hardness about 6–7 Mohs; coarse-grained with alternating pale quartz–feldspar and darker mica-rich bands, showing a dull to slightly vitreous luster and weak foliation. No obvious cleavage as a whole rock, though mica layers cleave readily.
Formation & geological history
Formed when granite or other felsic rock underwent intense regional metamorphism, producing mineral segregation and banding during high heat and pressure. Such rocks commonly formed deep in Precambrian to younger continental crust.
Uses & applications
Used as dimension stone, paving, aggregate, landscaping rock, and architectural facing; attractive banded pieces are collected as decorative specimens. Typical rough specimen value is about $5–$30, or $30–$100+ for unusually colorful polished pieces.
Geological facts
The specimen’s irregular pale-purple, gray, tan, and dark bands strongly fit gneiss, though exact identification would require a fresh surface or testing. It may locally grade into migmatite or weathered granitic gneiss.
Field identification & locations
Look for coarse visible crystals and compositional banding that remains continuous through the stone; unlike slate, it is not fine-grained, and unlike ordinary granite, it has pronounced mineral layering. Common in ancient shield regions and major mountain belts worldwide.
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