Here's why:

* Amethyst is a type of quartz: Quartz is a very common mineral found in various forms within the Earth's crust.

* Formation: Amethyst forms when quartz crystals are exposed to trace amounts of iron during their formation. This exposure gives the quartz its characteristic purple color.

* Geological Locations: Amethyst is often found in cavities and veins within volcanic and metamorphic rocks. These rocks form deep within the Earth's crust and are brought to the surface through geological processes.

So, while amethyst doesn't form at the very surface, it is definitely a mineral found within the Earth's crust.