A medium- to coarse-grained metamorphic rock, gneiss exhibits alternating light and dark bands, which is a characteristic usually caused by the separation of mafic and felsic minerals. However, despite their foliated appearance, gneiss does not exhibit the well-developed cleavage and schistosity that is found among schists. The light bands of the rock are typically composed of quartz and feldspar, but the composition of the dark bands fluctuates considerably. Variable in a number of other regards as well, gneisses are often classified based upon a number of attributes, including parent material, chemical composition, and probable formation process, resulting in descriptors such as diorite gneiss, diopside gneiss, and orthogneiss.