Here's why:

* Limestone is primarily composed of the mineral calcite (calcium carbonate). While it can contain minor amounts of other minerals, the vast majority of its composition is calcite.

* Other rocks are typically polymineralic, meaning they are composed of multiple minerals. For example, granite is composed of quartz, feldspar, and mica.

Important note: While limestone is the most common monomineralic rock, it's not the only one. Other examples include:

* Rock salt (halite)

* Rock gypsum (gypsum)

* Quartzite (almost entirely quartz)

Let me know if you'd like more information about any of these rocks!