Bad quality goods

I bought my mother a pendant chain at your Bayside Mall, Cape Town store for Christmas. It was obviously gifted to her on Christmas day and worn for about a month and its started turning black. I subsequently lost the receipt and therefore resided myself to leaving it as is and purchasing another gift but then realised I have my bank statement as proof. I was assured by the sales assistant that the tree of life pendant and chain was sterling silver and yet it didn't last long before it turned black. Why are you selling items for such high prices and then the quality of said items is ****. I've wasted my money and embarrassed myself with this gift.