Sterns sold us an engagement ring that wasn't real gold.

In 2021, we bought an engagement ring from Sterns for almost R7k but could not take it immediately because it needed to be sent away for resizing. We received it back after a bit of a delay and thought nothing of it until we wanted to make some changes to the ring recently and the jeweller told us that the ring wasn't real gold. We showed him the receipt for the payment we had made and told him it had to be because we bought it from a well-known retailer but the acid drop that they use to test whether something is real gold or not, turned a horrible shade of green. It was awful. I cannot believe that this has happened and that a trusted retailer could sell **** gold items. I'm disgusted and disappointed because I don't think I have any recourse here.