metamorphic
Banded gneiss pebble
Gneiss, commonly quartz–feldspar–mica bearing
AI-generated identification · may be incorrectHardness about 6–7 Mohs; gray, white, and dark bands; generally dull to slightly glassy luster; interlocking granular crystals with foliation, but no prominent cleavage as a whole. The rounded shape indicates water or abrasion polishing.
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Physical properties
Hardness about 6–7 Mohs; gray, white, and dark bands; generally dull to slightly glassy luster; interlocking granular crystals with foliation, but no prominent cleavage as a whole. The rounded shape indicates water or abrasion polishing.
Formation & geological history
Formed by high-grade regional metamorphism of granite or other sedimentary/igneous protoliths, commonly during ancient mountain-building events; the exact geological age cannot be determined from the photograph.
Uses & applications
Used as dimension stone, facing stone, landscaping rock, aggregate, and decorative polished material; attractive banded pieces are collected but are usually modestly valued.
Geological facts
The alternating light and dark layers reflect mineral segregation during metamorphism—typically quartz/feldspar-rich bands alternating with biotite- or amphibole-rich bands.
Field identification & locations
Identify it by strong compositional banding, crystalline texture, and hardness; unlike layered sedimentary rock, its bands are recrystallized and commonly contorted. Gneiss occurs in Precambrian shields and metamorphic mountain belts worldwide.
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