Corals are actually the skeletal remains of marine invertebrates. They live in compact colonies of many identical individual polyps.The fleshy skin polyps secrete a calcium carbonate to form a hard skeleton from which coral grows like branches and trees in the ocean.
When buying corals one has to be able to identify fakes and natural corals. The safest is to obtain a lab test to authenticate the corals. Especially if you are purchasing expensive corals.