Repair cost unfair

On the 29th December 2014 I bought a engagement ring set from the American Swiss in the Pavilion shopping centre. In June 2015 one of the stones fell out of the ring. Towards the end of June two more stones fell out of the ring. When I went to the American Swiss on the bluff with the rings and the receipt to see if they would repair it, I was told by the manager that it would cost me about R150 per stone and labour to fix the rings. That is about how much I paid for the ring set. Surely this is bad workmanship as I didn't do anything to cause this to happen and considering that the rings were only 6 months old. This was not an expensive ring set but not all of us is rolling in the money. Another point is that I got married at the end of February and the wedding band has only been woren since then. That makes it 4 months and stones falling out isn't acceptable.