What substance makes up the coral skeleton?
A coral reef is composed of calcium carbonate or limestone.
This is absorbed from the water by colonies of coral polyps and coralline algae. Most of the underlying foundation of the reef is dead, made up of layer upon layer of coral skeletons. The living reef is built over the top of this, by tiny coral polyps. Thus adding new limestone to the massive base structure.
The polyps make skeletons of calcium carbonate around themselves. These are called corallites. Several algae, namely zooxanthellae inhabit corals and are the reason for the beautiful colour of the corals.