Here's why:

* Formation: Limestone forms primarily from the accumulation of the shells, skeletons, and other hard parts of marine organisms like clams, snails, corals, and microscopic creatures called foraminifera.

* Composition: These shells are largely made of calcium carbonate (CaCO3), which is the primary mineral in limestone.

* Types: There are various types of limestone, depending on the dominant organism contributing to its formation:

* Coquina: Limestone made up of easily identifiable shell fragments.

* Fossiliferous Limestone: Contains identifiable fossils.

* Chalk: A fine-grained limestone primarily formed from the accumulation of microscopic marine organisms.

Let me know if you have any other questions about rocks!