Gneiss is a coarse to medium grained banded metamorphic rock formed from igneous or sedimentary rocks during regional metamorphism. Rich in feldspars and quartz, gneisses also contain mica minerals and aluminous or ferromagnesian silicates. In some gneisses thin bands of quartz feldspar minerals are separated by bands of micas; in others the mica is evenly distributed throughout. Common orthogneisses (gneisses formed from igneous rocks; those formed from sedimentary rocks are called paragneisses) are similar in composition to granite or granodiorite, and some may have originally been lava flows. Augen gneiss is a variety containing large eye shaped grains (augen) of feldspar. Injection gneisses are formed by injection of veinlets of granitic material into a schist or some other foliated rock. Banded gneisses called migmatites are composed of alternating light colored layers of granite or quartz feldspar and dark layers rich in biotite. Some migmatites were formed by injection, others by segregation of quartz and feldspars.

The origin of a gneiss can usually be determined by its chemical composition and mineral content. A distinction between gneiss and schist is difficult to draw, for many gneisses look far richer in mica than they are, when mica rich parting plane is seen.