I would not expect a real diamond for fifteen bucks either but if it was listed as being real and was in the fine jewellery category they would definitely get a neg from me!

Far too many sellers are getting away with inaccurate and miss categorised listings and as ebay are slow to act, if they ever do, about the only way to discourage them is to leave negs.

I do not have a blanket refund on return policy as it just encourages buyers like yours, I promise a full refund including return post if I am at fault but for change of mind a refund of item price only on return. Unless I receive incontrovertible proof in the form of a photo or condition report  I would fight any Paypal dispute to the bitter end and I haven't lost one yet!