Mailing a diamond ring is complicated. No one will sell you insurance. I have a headache, so I'm not even going to go into why this has come up, but don't worry, it was boring anyway.
Say you did mailed or FedExed or UPSed your engagement/wedding ring. And say they lost it.
a) replace it with something that looked exactly the same
b) something the same only bigger
c) choose something altogether different
d) not bother since nothing can replace the original