Here's why:

* Formation: Agates form within cavities in volcanic rocks (igneous) or within sedimentary rocks. These cavities are often created by gas bubbles in lava or by the dissolving of fossils.

* Process: As groundwater rich in silica (SiO2) seeps through these cavities, it deposits layers of silica. These layers are often banded and create the distinctive patterns we see in agates.

* Metamorphism: While agates can be found in metamorphic rocks, they are not formed through metamorphism themselves. The silica layers within the agate are deposited before any potential metamorphic events.

So, although agates are often found in association with igneous and metamorphic rocks, their formation process classifies them as sedimentary rocks.