Heliopora coerulea
Diagnosis
Often brown and don't appear blue at all. Secondly, although they have a hard skeleton, they are not grouped with other hard corals.
Colonies 15-30cm, polyps about 0.5cm. Their internal skeletons are blue, hence their common name. The blue colour is due to the iron salts that are incorporated into their skeletons. On the outside, they are usually brown because the thin layer of brownish living tissue that covers the outer surface of the skeleton. The skeletons are made of a different kind of calcium carbonate (fibro-crystalline argonite) that is more brittle than that of true hard corals. Inside the skeleton are tubes where the long, thin polyps live and a system of canals. Blue coral colonies are usually boulder shaped with knobs. They may also have thick leaf-like forms or columns, and may even be encrusting or plate-like.