Gneiss is a metamorphic rock with color banding. Regional metamorphism occurs at convergent plate margins where plates are colliding with one another. These rocks may be produced by regional metamorphism , but no foliation is developed because platy or sheet-like minerals do not form, neither do alternating light and dark bands of silicate minerals. These rocks may also be produced by contact metamorphism . Metamorphic rocks are classified on the basis of the presence or absence of foliation, together with the size of the crystals.
Composition is important in the classification of the non-foliated metamorphic rocks.