Here's why:

* Formation: Agate forms when silica-rich fluids seep into cavities in existing rocks, often volcanic rocks like basalt.

* Transformation: The silica crystallizes within these cavities, creating the distinctive banded patterns we see in agate. This crystallization process is a metamorphic change, where the original rock is altered by heat, pressure, and chemical reactions.

While agate often originates from volcanic rocks (igneous), it's the metamorphic process that creates its unique appearance and structure, making it a metamorphic rock.