@morichiro technically any shelled mollusk can produce pearls and the type of pearl is based on the species. So bases on it's size likely something like a giant clam.
@metincan3606 To be honest, he probably wouldn't have to open it. Just put it in his inventory and he'd be able to see the contents.
It's a case of Schrödinger's Cat, but viewed through quantum time-space magic.
The merfolk should have been marketting that stuff. And if not that, then they should have used the beaches as dumps for that stuff, in-so-far as the beaches would regularly have been cleaned by H. sapiens.