We may never know what this round object is because the Blue Reef Aquarium (Portsmouth, UK) whom the fisherman donated the oyster to has decided to leave the fossil intact.

Could you show the same restraint and resist the lure of possible treasure?

Right: MRI scan of the oyster fossil shows a golf-ball sized round object where pearls are usually found.  A pearl of this size is twice as large as the largest pearl known to man.

[via DailyMail]