metamorphic
Gneiss boulder
Banded gneiss, commonly quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectTypically gray with strong light–dark compositional banding and folded foliation; quartz and feldspar give a vitreous to dull luster, while mica may sparkle. Hardness is about 6–7 Mohs, with poor schistose cleavage and high durability.
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Physical properties
Typically gray with strong light–dark compositional banding and folded foliation; quartz and feldspar give a vitreous to dull luster, while mica may sparkle. Hardness is about 6–7 Mohs, with poor schistose cleavage and high durability.
Formation & geological history
Formed when granite or other crustal rocks underwent intense regional metamorphism involving high temperature and pressure, commonly during Precambrian to Paleozoic mountain building. The rounded shape reflects glacial transport and/or river abrasion.
Uses & applications
Used as dimension stone, landscaping stone, riprap, aggregate, and occasionally polished architectural stone; this specimen is primarily decorative or educational.
Geological facts
The swirled bands record ductile deformation deep within the continental crust. Gneiss is identified by mineral segregation into bands rather than the thin, flaky layering typical of schist.
Field identification & locations
Common in ancient continental shields and metamorphic mountain belts, including the Rocky Mountains and Canadian Shield; inspect a fresh surface for interlocking quartz, pale feldspar, dark biotite/hornblende, and foliation. Exact mineral identification requires a hand lens or thin section.
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