Defective Product

Would like to a a picture but don't know how. So May2021 i bought a neckless/earings combo, my daughter worn it 3times, and it turn ugly black! Took it back to store & they told me its because its gold/silver plated & its not suppose to lay in the box! Huh? Why u wrapped & sell jewelry in a box but at home u not suppose to keep it in said box? Anyhow, they send products away for testing, products returned cleaned, guess they cleaned it. So this morning i got a phone call from a sales lady at @Sterns informing me that I had to pay for the repairs of products, according to her products was send for test against body accid??? Question, against whose acid did they test the neckless??? Never was i told that I was going to pay for services on a product that was worn 3times& was less than 3months old when I return it. I wanted my money back but as it was a gift to my daugter& she wanted to keep it. People beware before buying at stern, this is not on