Here's why:

* Igneous rocks are formed from the cooling and solidification of magma or lava.

* Sedimentary rocks are formed from the accumulation and cementation of sediments.

* Metamorphic rocks are formed when existing igneous or sedimentary rocks are subjected to intense heat and pressure deep within the Earth's crust. This process changes the rock's mineral composition, texture, and sometimes even its chemical composition.

Examples of metamorphic rocks:

* Marble (from limestone)

* Slate (from shale)

* Gneiss (from granite or other igneous rocks)

* Quartzite (from sandstone)

So, while the original rock might have been igneous or sedimentary, the transformation by heat and pressure creates a completely new type of rock: metamorphic.