I purchased a ring in September 2023 to surprise my wife on her anniversary

I purchased a ring in September 2023 to surprise my wife on her anniversary.The ring was beautiful and valued R4999.00.After 5months the ring changed colour.I took it back to sterns south coast mall.They sent it for cleaning it came back with a shine again.A month later the ring changed colour again.Now my money is gone to waste as the ring can't be used.How is sterns going to sort this out as its not my fault the ring changed colour.As a customer I feel I have been ********** by such a huge company.How do I get my money back or another ring?