"The polyps that build the reefs are known as stony corals.  They live together in colonies.  Cells on the outside of the polyps' bodies collect a chemical called calcium carbonate from the sea water.  Calcium carbonate hardens to form limestone, and this grows into a protective shell around each polyp.  When the polyps die, their shells remain.  As new polyps grow on the old limestone shells, the reef becomes larger."