Protolith is the original, sedimentary rock from which a metamorphic rock is formed. It refers to the precursor lithology of a metamorphic rock. For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology. Sedimentary rocks have no protolith because they are made of sediment. Igneous rocks have no protolith because they form from magma.