Invertebrates, or animals without backbones, don't have skeletons as we know them. But some have outer skeletons, or exoskeletons, like the hard shells of crabs and lobsters. Coral is made of the exoskeletons of many thousands of tiny marine animals called polyps. The hard coral is similar to limestone and may form enormous reefs. The Florida Keys are islands made of coral.
Fossil corals are even harder.
Silica dissolved in seawater can harden, replacing the limy corals with a form of quartz mineral called chalcedony. The process can take millions of years. Scientists say the coral has become agatized. It looks pretty much the same, but is made of something different. Unofficial Fossil State Gem .