OP
I don't think so you were scammed. It is a human error and such kinda of errors happen very rare. Agreed that it was a mistake from JA but they are honest enough to accept their mistake and apologize for it. In your post you have mentioned that compared to other diamonds with almost similar specs the one you got is at a good price, then why worry. Place yourself in their shoes and ask your self what will you do?
I have no intention to hurt your feelings or please don't feel offended. JA wants your business and loosing one cust is like loosing 100's of cust, at least they are being honest unlike other vendors they are not giving you false reasons like "Diamond is chipped", "diamond is lost in transit" etc etc...