Antipatharia

You live in the sunless seas of the deep ocean, and a few shallow but sunless sounds.

You grow very slowly, but you live a very long time indeed.  Some of you are known to be over four thousand years old.

Your outer shell grows thick, and hard, and dark.  Inside, your cnidarian colours are very bright indeed.

Your shell is sought after by humans as it can be polished to a brilliant shine, and is hard to get.

You are a Stony Coral.