Coral reefs are made of huge amounts of calcium carbonate (CaCO3), or limestone, that is deposited by organisms. Of the thousands of species in coral reef communities, only a fraction produce the limestone reef. The most important of these reef building organisms, as you might guess, are corals.
Coral is a general term for several different groups of cnidarians, only some of which help build reefs. In reef-building, or hermatypic, corals the polyps produce calcium carbonate skeletons. Billions of these tiny skeletons form a massive reef. The most important reef builders are a group known as scleractinian corals, sometimes called the stony or “true” corals.