Here's why:

* Minerals are the building blocks of rocks. Minerals are naturally occurring, solid substances with a definite chemical composition and crystal structure. They are formed through various geological processes.

* Rocks are aggregates of minerals. Rocks are formed by the accumulation, cementation, or crystallization of minerals.

* Exceptions exist. Some rocks, like obsidian (volcanic glass) or coal (formed from organic matter), are not primarily made up of minerals.

Therefore, while exceptions exist, it's safe to say that the vast majority of rocks are composed of a combination of different minerals.