Limestones are often of biochemical origin. This means they are formed from the accumulation and cementation of the remains of organisms, primarily marine organisms with hard parts like shells, skeletons, and corals. Here's a breakdown:

* Key Organisms: The most common contributors to limestone formation are:

* Marine invertebrates: These include organisms like mollusks (clams, snails), brachiopods, echinoderms (sea urchins, starfish), and corals.

* Algae: Certain types of algae, like coccolithophores, produce microscopic calcium carbonate plates that contribute significantly to limestone formation.

* The Process:

* Accumulation: Over time, the shells and skeletons of these organisms accumulate on the ocean floor.

* Cementation: As more sediment builds up, the weight compresses the organic material, and dissolved minerals in the water (mainly calcium carbonate) act as a cement, binding the fragments together.

* Lithification: This process of compaction and cementation transforms the loose sediment into solid rock, forming limestone.

Other Origins of Limestone:

While biochemical limestone is the most prevalent, there are also other origins:

* Chemical: Some limestone forms directly from the precipitation of calcium carbonate from water, often in warm, shallow marine environments.

* Organic: Limestone can form from the accumulation of plant debris, particularly in freshwater settings.

So, in summary, the majority of limestones are indeed formed through biochemical processes involving the remains of marine organisms.