More specifically, it's a metamorphic rock. Amethyst is a variety of quartz that gets its violet color from trace amounts of iron and other impurities. The quartz crystals form within the earth's crust, often in volcanic rocks, and are then exposed by erosion.
Here's the breakdown:
* Igneous rocks are formed from the cooling and solidification of magma or lava.
* Metamorphic rocks are formed when existing rocks (igneous, sedimentary, or other metamorphic rocks) are transformed by heat, pressure, or chemical reactions.
While amethyst quartz can be found within igneous rocks, it is itself classified as a metamorphic rock due to the process that creates its distinct purple color.