Here's why:

* Hard substance: Rocks are generally solid and resistant to deformation.

* Composed of one or more minerals: Rocks are formed by the aggregation of minerals, which are naturally occurring, solid, inorganic substances with a defined chemical composition and crystal structure.

* Mineral-like substances: Sometimes, rocks can also contain materials that are not true minerals, like volcanic glass or organic matter.

Let me know if you'd like more details about rocks and their formation!