These whales have a tough belly that diamond head dolphins love to smash.
They end up developing ore spots along their body which spread more as they age and grow.
If their bellies are broken into, the whale usually dies. Within its belly is a colorful lard that burns many different colors - Which is sometimes used for fireworks. It's illegal to kill a rockbelly whale without using ALL of its body, however, and fines are harsh when the law is breached.
Also...it seems there are rockabelly whales that have started communicating - or so reported, making killing them even more tragic and frowned upon by those in the Luxuries.