metamorphic

Gneiss

Banded quartz–feldspar–mica gneiss

AI-generated identification · may be incorrect

Coarse-grained, dark gray to tan rock with irregular light and dark mineral bands; typically quartz, feldspar, biotite, and hornblende. Hardness is about 6–7 Mohs, with vitreous to pearly luster and weak-to-strong foliation rather than distinct cleavage.

Identified September 12, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when granite or other rocks undergo intense heat and pressure deep in the continental crust, commonly during Precambrian to Paleozoic mountain-building events. The apparent banding reflects mineral segregation during high-grade metamorphism.

Uses & applications

Used as dimension stone, paving, landscaping rock, aggregate, and occasionally as decorative building stone; generally not a gemstone.

Geological facts

Gneiss is among the oldest exposed continental rocks and may preserve evidence of ancient mountain belts and continental collision.

Field identification & locations

Identify it by coarse interlocking grains and alternating light quartz–feldspar-rich and dark biotite–amphibole-rich zones; unlike granite, its minerals are commonly aligned or streaked. Common in continental shield regions such as Canada, Scandinavia, the Alps, and the Appalachian Mountains.