It's actually the hard, colorful calcareous skeleton secreted by certain marine polyps
Thin, calcareous septa (sclerosepta), which provide structural integrity, .... 
corals may still exist on hard substrates, but their capacity to secrete... [DOC] [TLE] The Great Barrier Reef by WILLIAM LAUER on Prezi [PAR] hard, variously colored, calcareous skeleton secreted by certain marine polyps 
coral is also important because it helps control the levels of carbon dioxide in the
... [DOC] [TLE] Corals - Aquafind [PAR] They have living part called polyp that secrete a calcareous skeleton.