metamorphic
Gneiss
Banded gneiss; typically quartz–feldspar–mica assemblage
AI-generated identification · may be incorrectHardness about 6–7 Mohs; gray, cream, and dark bands with a coarse crystalline texture, vitreous to dull luster, and strong compositional foliation but no consistent cleavage. The layered appearance reflects alternating quartz/feldspar-rich and mica/amphibole-rich minerals.
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Physical properties
Hardness about 6–7 Mohs; gray, cream, and dark bands with a coarse crystalline texture, vitreous to dull luster, and strong compositional foliation but no consistent cleavage. The layered appearance reflects alternating quartz/feldspar-rich and mica/amphibole-rich minerals.
Formation & geological history
Forms when granite or other crustal rock undergoes high-grade metamorphism, commonly at roughly -failed-650–800°C and substantial pressure, causing mineral segregation and recrystallization. Most examples are Precambrian to Paleozoic, though gneiss forms in many active mountain belts.
Uses & applications
Used as dimension stone, facing stone, paving, landscaping material, and aggregate; attractive polished pieces are collected but are generally low-value.
Geological facts
The specimen’s streaked, folded bands and coarse grains strongly suggest gneiss, though the photograph cannot establish its exact mineral variety. It may also be a foliated granitic or migmatitic rock.
Field identification & locations
Common in ancient continental shields and mountain cores, including Canada, Scandinavia, India, the Alps, and the Appalachians. Identify it by coarse interlocking crystals and compositional banding; unlike slate or schist, its bands are usually thicker and less evenly fissile.
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