Older Than Time.
What we call "hard or stony corals" are actually skeletons of small marine animals called coral polyps, which grow in groups or "colonies". As polyps die, new ones grow on top of their skeletons, very slowly building coral reefs. With some species needing 250 years to build one meter of coral reef, it is believed that some of the world's largest reefs started forming more than 400 million years ago.