Here's why:

* Mineral: A mineral is a naturally occurring, solid, inorganic substance with a defined chemical composition and a specific crystal structure.

* Rock: A rock is a solid aggregate of one or more minerals.

Agate is formed from the cryptocrystalline (very finely crystalline) variety of quartz. It is a banded rock, meaning it has layers of different colors and patterns. The colors come from impurities within the silica.

So, while agate is composed of the mineral quartz, it is itself considered a rock due to its banded structure and the presence of other minerals in small amounts.