Here's why:

* Igneous rocks form from the cooling and solidification of molten rock (magma or lava).

* Agates form within cavities in volcanic rocks, often within geodes. These cavities were originally filled with gas bubbles or water pockets.

* The process: As the volcanic rock cools, silica-rich water percolates into the cavities. Over time, this silica precipitates out, layer upon layer, forming the distinctive banded patterns that characterize agates.

In essence, agates are formed by the deposition and crystallization of minerals within pre-existing cavities, a process that defines sedimentary rock formation.