Faulty Engagement Ring

Hi! We got engaged last Feb and he proposed with a temporary ring, so we ordered a new one in March. In September one of the prongs snapped and the diamond fell out. Luckily I found the diamond, contacted the store, and they repaired it no problem. Yesterday, the diamond fell out again as one of the prongs had snapped. I was just at home sitting on the sofa, I hadn't knocked it or anything like that, I just looked down and the diamond had fallen out. I've emailed the store again and told them I didn't really want it repairing as I'm wary that this will only happen again, and basically all they have said is that I will need to pay for this to be repaired this time and that they will give me a quote for this once I've sent it back to them. I haven't even had it 12 months and this has happened twice already. I'm really careful with it (especially after it already happening once), I never wear it for bed, I take it off when I shower etc, and I work at a desk all day so I don't do anything that would break it. Can anyone advise on what I can do, as I'm just not sure what my rights are with something like this. Thank you.