This is what you get when you don’t thoroughly clean every crevasse on your body.
You end up pulling years-old gunk from your belly button. I don’t know if that’s an accurate description of what’s being pulled from this person’s belly button, but having a navel stone does not sound pleasant.
As a kid, I always thought people who had an “outtie” were weird. They’ll get the last laugh in this case because there’s no crevasse to be had on that front. Good luck getting a navel stone in that.