Here's why:

* Igneous rocks are formed from the cooling and solidification of molten rock (magma or lava). Amethyst doesn't form directly from this process.

* Metamorphic rocks are formed when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are transformed by heat, pressure, or chemical reactions. Amethyst forms when the mineral quartz (which can be found in igneous or sedimentary rocks) is exposed to these conditions.

Specifically, amethyst is a variety of quartz that gets its violet color from trace amounts of iron and other impurities. The heat and pressure involved in its formation cause the quartz to crystallize in a specific way, leading to its characteristic purple hue.