Gneiss is a medium- to high-grade foliated metamorphic rock displaying a coarse-grained banding (also known as gneissose structure). ‘Gneiss’ derives, indeed from the German gneist, ‘spark’, likely a reference to the presence of large grains that reflect light. In structural terms, banding is a foliation with a spacing larger than 1 cm, consisting of coarse-grained minerals that we well visible with the naked eye. A rock which is more fine-grained and more pervasively foliated is a schist (representing also a lower grade metamorphic rock, compared to gneiss). In gneissic rocks, banding is usually an alternation of light-colored layers (typically containing quartz and feldspars) and dark-colored layers (containing biotite, amphibole, garnet etc.). This gneissose structure, or banding, is very rarely a reflection of original sedimentary bedding. Rather, it results from a complete reorganization of the original protolith into a metamorphic fabric through deformation and differentiation of minerals with different composition. The term gneiss has a purely structural connotation (i.e. does not give information about composition) and it is preferred to include the most important metamorphic minerals present as a suffix (e.g. garnet-staurolite gneiss).
Some gneisses show lens- or eye-shaped grains, called augens (from the German word for ‘eye’). The related rock term for a gneiss containing such grain is augen gneiss. A gneiss can also display a strong linear fabric, defined by elongated, coarse-grained mineral grans. In this case, the rock is a lineated gneiss (or pencil gneiss). It is possible to distinguish gneisses based on their protolith: a paragneiss derives from the metamorphism of sedimentary rocks, whereas an orthogneiss from igneous ones. The recognition of the protolith is based on the presence of diagnostic relic structures/grains, rock chemistry, and/or metamorphic minerals.
Since gneiss forms up to very high metamorphic grade, it can possibly experience partial melting. This leads to the formation of migmatites, rocks with relic metamorphic structures and crystallized melt. It is not possible to recognize migmatites based on field data alone, as they resemble many high-grade rocks, including gneisses. A gneiss that experienced partial melting can be called migmatitic gneiss.
Gneiss – Sandatlas.org
Rocks and Minerals in Thin Section – a Colour Atlas