Gneis

Gneis, in English commonly written as gneiss, is a high-grade metamorphic rock distinguished by gneissic banding, in which light and dark mineral layers alternate. The light bands are typically rich in quartz and feldspar, while the dark bands contain biotite, amphibole and other mafic minerals. The mineral layers are well developed, giving the rock a visibly striped or foliate appearance that reflects directional pressure during formation.

Gneis forms under high-temperature and high-pressure conditions during regional metamorphism, usually from preexisting rocks such as granitic plutons, diorites, or sedimentary rocks like sandstones and shale. Heat and directed stress cause minerals to recrystallize and segregate into different phases, producing the characteristic banded texture. Some gneisses arise by partial melting and resettling as migmatite, but distinct gneiss remains identifiable.

In terms of classification, gneiss can be felsic (light-colored) or mafic (darker) depending on its mineralogy, and many varieties are named after their dominant minerals. Gneiss is common in continental crust and is associated with major orogenic belts such as the Grenville, Alps, and Himalayas.

The term gneiss derives from the German word Gneis. In geology, it denotes a metamorphic rock with pronounced banding rather than a specific mineral composition. Gneis is used as a decorative and building stone in some regions and serves as a record of the geological history of mountain belts.