"Unbreakable" wedding ring

August last year, I went on a search for the perfect wedding band for my fiancé. I found it at Belgravia Jewellers. The salesman said that it is unbreakable and it will keep him a lifetime without damage and scratches. Even if he looses weight he can only come and exchange it. December my husband washed his hands and the ring slipped and fell. It chipped. We took it back to them and they said they will send it to the manufacturers whom will replace it or fix it. After lot of queries from my side Belgravia let me know today that the manufacturers said they don't cover the fact that it broke (chipped) and that I need to buy a new ring. I don't understand how it became my responsibility after they sold the ring to me under false pretences. Please can you help me?