Corals form limestone from "summary" of Principles of Geology, Volume 1 by Charles Lyell,Sir Charles Lyell

Corals produce abundant calcium carbonate, a mineral that provides the base materials from which limestone is formed. The presence of limestone in different areas of the world suggests that coral reefs have been around for a long time.

Calcium carbonate is deposited when living corals die and become skeletons, forming visible indications of previous coral populations within the rock face. Over hundreds or even thousands of years, these layers build up to form layers of limestone.

Limestone formations such as coral reefs are found scattered across the seabed, nowadays often covering sunken lands and past physiographic features. It thus gives insight into how parts of the world may have looked millions of years ago.

Evidence derived from observational analysis shows that coral reefs are bioherms which accumulate sedimentload on their framework due to their ability to quickly construct reef structures at places where sedimentation occurs.

Many types of fossils including coral remains can be found embedded deep within chalky limestone beds, raising the possibility of uncovering evidence of ancient life on Earth.

Deposition of sediments is aided by slow subsidence of sediment grains onto the substrate, forming an outcropping of solid lime-rock otherwise known as limestone. This is then cemented through the deposition of calcium carbonate layers via calcification of other marine organisms like algae and oysters.