Searching Costco.com, I came across a $329,999 yellow diamond piece. Of course, it had to be purchased over the phone. I assume this would require a money transfer, and not a credit card purchase. No one will sell you something that expensive with a credit card.

I would assume that the same thing goes for anything over $100,000 (they will allow you to purchase a 99,999 diamond online, but not a $119,000).