How can I assure the customer that the stone they mailed me is the
stone they received back.
Just my personal response, but no way would I send you a diamond to
be mounted. I have no idea who or what you are, but the business you
(we) are in depends on trust.
I can’t think of a method to assure another that the stone they sent
to you is the stone that will be returned.
There are lots of internet scams. Face to face instills confidence in
a way that the procedure you propose doesn’t.