Here's why:

* Rocks are made of minerals. Minerals are naturally occurring, solid substances with a defined chemical composition and crystal structure. Rocks are formed from aggregates of one or more minerals.

* Even seemingly "non-mineral" rocks contain minerals. For example, sandstone is made up of tiny grains of quartz (a mineral), and limestone is primarily composed of the mineral calcite.

So, no matter what kind of rock you're looking at, it's made up of minerals!