So then what is the cause?

Maybe because the demand for rings is not for their commodity value, but for their signaling value? So buying a ring from Tiffany's is worth a lot more than buying one off craiglist? And they aren't scarce so jewelers don't need to snatch them up off the "open market", but they need to make high margins to cover the overhead of their fancy stores.

Are you looking for a cause to the "absurdly large" degree? Or just pointing it out as some mysterious conspiracy?