Limestone is primarily formed from biogenic (organic) material, specifically the calcium carbonate skeletons of marine organisms like:

* Corals

* Mollusks

* Foraminifera

* Algae

These organisms extract calcium carbonate from seawater and use it to build their hard parts. When they die, their skeletons accumulate on the ocean floor and are eventually cemented together by pressure and chemical processes, forming limestone.

While limestone can be formed through chemical precipitation of calcium carbonate in some cases, the vast majority of limestone is biogenic in origin.