metamorphic
Banded gneiss pebble
Gneiss, commonly quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectTypically gray and tan, with alternating light and dark compositional bands, granular texture, and vitreous to dull luster. Hardness is about 6–7 Mohs; crystals are interlocking, with no prominent cleavage as a whole, and specific gravity is roughly 2.6–2.8.
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Physical properties
Typically gray and tan, with alternating light and dark compositional bands, granular texture, and vitreous to dull luster. Hardness is about 6–7 Mohs; crystals are interlocking, with no prominent cleavage as a whole, and specific gravity is roughly 2.6–2.8.
Formation & geological history
Formed when granite or other crustal rock undergoes intense heat and pressure, causing mineral segregation into gneissic banding. Most examples are Precambrian to Paleozoic, though gneiss forms during many mountain-building events.
Uses & applications
Used as dimension stone, architectural cladding, landscaping material, aggregate, and decorative specimens; attractive polished pieces have modest collecting value.
Geological facts
The visible banding records metamorphic deformation and mineral separation, not sedimentary layering. Light bands are usually quartz and feldspar, while darker bands commonly contain biotite, amphibole, or pyroxene.
Field identification & locations
Identify it by examining the coarse, interlocking grains and persistent alternating bands; it is common in ancient continental shields and metamorphic mountain belts. The pictured rounded form is likely glacial or stream-worn.
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