metamorphic

Gneiss

Banded gneiss; quartz–feldspar–mica rock

AI-generated identification · may be incorrect

Typically 6–7 Mohs; light gray to pink/white with dark biotite- or amphibole-rich bands, vitreous to pearly luster, and coarse granular crystals. Strong compositional banding and foliation are diagnostic; cleavage follows mica layers rather than the whole rock.

Identified September 5, 2026More metamorphic →

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Physical properties

Formation & geological history

Forms when granite, sedimentary rocks, or volcanic protoliths undergo high-grade regional metamorphism, commonly at roughly 600–800°C and substantial pressure during mountain building. Individual specimens may range from Precambrian to younger Phanerozoic ages.

Uses & applications

Used as crushed aggregate, building stone, paving, and decorative slabs; attractive polished pieces are collected but are generally low-value.

Geological facts

The pictured specimen appears to be strongly foliated, banded gneiss, possibly with amphibolite or mica-rich inclusions. Gneiss commonly represents deep crustal rocks later uplifted and exposed by erosion.

Field identification & locations

Look for alternating light quartz/feldspar bands and dark mica/amphibole bands, coarse crystals, and foliation; unlike granite, its minerals are visibly segregated into layers. Common worldwide in ancient continental shields and metamorphic mountain belts.