Here's why:

* Minerals are naturally occurring, solid substances with a defined chemical composition and crystal structure. They are the building blocks of rocks.

* Rocks are solid aggregates of one or more minerals. They can also contain other materials like glass, organic matter, or volcanic ash.

For example:

* Granite is a rock composed of the minerals quartz, feldspar, and mica.

* Limestone is a rock primarily composed of the mineral calcite.

* Sandstone is a rock composed of sand grains, which are mainly made up of quartz.

There are exceptions, such as obsidian, which is a volcanic glass and not technically a mineral. However, the vast majority of rocks are composed of minerals.