The parent rock is a unique identifier for metamorphic rocks because these rocks are formed by the changes in the parent rock due to heat and pressure. These rocks are formed from the preexisting rocks that can be sedimentary rocks and igneous rocks. For example, metamorphosis of limestone makes marble which is a metamorphic rock. Identifying the parent rock is problematic because the parent rocks change a lot due to metamorphosis and only few minerals left can help in identification process.
