What is a coral reef made of?

A coral reef is composed of thin plates or layers of calcium carbonate secreted
over thousands of years by billions of tiny soft bodied animals called coral polyps.
It takes years for some corals to grow an inch and they range in size from a
pinhead to a foot in length. Each polyp excretes a calcereous exo-skeleton and
lives in a symbiotic relationship with a host algae, zooxanthellae, that gives the
coral its color. Zooxanthellae takes in carbon dioxide, process is through
photosynthesis, and then gives off oxygen as a by-product that is used by the host
polyp. Millions of polyps grow on top of the limestone remains of former colonies
to create the massive reefs. Yet these tiny animals form the only natural formation
visible from outer space.