The skeletal mass of the little
ocean creatures, which is largely
calcium carbonate, the same thing as in the mineral
calcite. Generally found in shallow waters with a temperature above 68 degrees Fahrenheit. It often has a branched,
tree-like shape in its natural form, although other types are found. The natural surfaces can be
sharp enough to cut waders and
divers who step on or rub against it.
Coral has been used as a gemstone and carving material for millennia. Most goverments now only allow coral that has broken off naturally from its base to be used for these purposes, to protect the reefs and the other sea animals that live in and around them. It is most commonly white or off-white, but yellow, pink, red, and black are found naturally (and tourist traps often have coral dyed all sorts of colors).
Coral jewelry is delicate; it can be harmed by acids in sweat and change color, so settings that keep the gem away from your skin are preferable. (Polishing with a small amount of hydrogen peroxide is supposed to help restore the natural color.)