metamorphic
Banded gneiss pebble
Gneiss, commonly quartz–feldspar–biotite gneiss
AI-generated identification · may be incorrectTypically medium- to coarse-grained, with alternating dark biotite/amphibole and light quartz–feldspar bands; hardness about 6–7 Mohs, nonmetallic luster, and no consistent cleavage as a whole rock. The rounded shape indicates abrasion as a stream or beach pebble.
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Physical properties
Typically medium- to coarse-grained, with alternating dark biotite/amphibole and light quartz–feldspar bands; hardness about 6–7 Mohs, nonmetallic luster, and no consistent cleavage as a whole rock. The rounded shape indicates abrasion as a stream or beach pebble.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent high-grade metamorphism, causing mineral segregation into bands through heat and pressure. Gneiss commonly forms in Precambrian continental crust, though younger examples occur in many mountain belts.
Uses & applications
Used locally as dimension stone, paving and landscaping aggregate, and decorative stone; attractive polished pieces are collected but are generally modestly valued.
Geological facts
The swirling bands record intense deformation and recrystallization rather than sedimentary layering. Dark bands are likely biotite or amphibole, while pale bands are mainly quartz and feldspar.
Field identification & locations
Identify it by distinct alternating mineral bands, a granular texture, and resistance to scratching by ordinary steel; unlike slate, it does not split into thin sheets. Common in glacial deposits, river gravels, beaches, and exposed ancient mountain rocks.
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