Many corals secrete an exoskeleton that builds up to become a coral reef.
Coral is calcium carbonate with a trade of carotene.
Calcium carbonate becomes a  reef.
A reef is made of
(1) it keeps an organism warm
(2) salt and water.
(3) granite
(4) magnesium
(5) gastropod shells
(6) ocean water
(7) calcium carbonate
(8) zinc