Here's why:

* Amethyst is a variety of quartz. Quartz is a mineral that forms in igneous rocks.

* Metamorphism alters existing rocks. The heat and pressure deep within the Earth can transform existing igneous rocks (like granite) into metamorphic rocks. This process can change the chemical composition and crystal structure of the original rock, leading to the formation of amethyst.

So, while amethyst originates from igneous rocks, it is classified as a metamorphic rock because it's formed through the metamorphic process.