Rai stones were these big discs carved from limestone and then polished and used as currency. It's not surprising to me that Tamatoa would collect them.
An interesting thing about these stones was they they were often too large to move conveniently. The ownership of large Rai Stones could "change hands" through oral tradition alone, with the stone staying in the same physical place. It is said that when a particular Rai Stone was lost at sea during transport the people decided that it still held its value for the owner.
I noticed another on his back but here are two examples.
by humdinger44