Here's why:

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

* Exceptions: There are a few exceptions to this rule. Some rocks, like coal, are primarily composed of organic matter. Other rocks, like obsidian, are made of cooled volcanic glass, which technically isn't a mineral because it lacks a crystalline structure. However, even obsidian often contains small amounts of minerals.

So, while there are a few exceptions, it's safe to say that the vast majority of rocks are made of minerals.