metamorphic
Banded gneiss pebble
Gneiss, commonly quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectTypically 6–7 Mohs; gray, cream, and dark bands with a granular texture and dull to slightly glassy luster. Composed mainly of quartz and feldspar with biotite or hornblende; the banding reflects mineral segregation during metamorphism.
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Physical properties
Typically 6–7 Mohs; gray, cream, and dark bands with a granular texture and dull to slightly glassy luster. Composed mainly of quartz and feldspar with biotite or hornblende; the banding reflects mineral segregation during metamorphism.
Formation & geological history
Formed when pre-existing igneous or sedimentary rock underwent intense heat and pressure, producing compositional layering and recrystallization. Individual gneiss bodies commonly formed during Precambrian to younger mountain-building events.
Uses & applications
Used as dimension stone, landscaping rock, paving, and aggregate; attractive polished pieces are collected as decorative specimens. This small rounded example has modest commercial value.
Geological facts
The irregular light-and-dark layers are gneissic banding, not sedimentary bedding. Its rounded shape indicates abrasion during transport in a river, beach, or glacial environment.
Field identification & locations
Identify it by coarse interlocking grains and alternating light quartz–feldspar bands with dark mica/amphibole bands; unlike slate, it does not split into thin sheets. Common in ancient continental shields and metamorphic mountain belts worldwide.
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