Here's why:

* Minerals are naturally occurring, inorganic solids with a definite chemical composition and crystal structure.

* Rocks are solid, naturally occurring aggregates of one or more minerals, or mineraloids.

Agate itself doesn't have a single chemical formula. It's composed of cryptocrystalline silica, which means the silica crystals are too small to be seen with the naked eye. The specific chemical composition of agate can vary depending on the impurities present, resulting in the beautiful banded patterns often seen in agate.

Since agate is formed from a combination of minerals (specifically silica), it fits the definition of a rock.