Here's why:

* Rocks are made of minerals: Minerals are naturally occurring, solid substances with a specific chemical composition and crystal structure.

* Mineral composition determines rock type: The types and proportions of minerals within a rock define its classification. For example, granite is made up of quartz, feldspar, and mica, while limestone is primarily made of calcite.

So, when you talk about the "chemical makeup" of a rock, you're essentially describing the minerals it contains and their relative abundance.