The buyer only bought the ring a couple of weeks ago - it would be reasonable to expect that the stone would not fall out in such a short time, especially as the ring was new.
Although you are not to blame, you are the seller and Ebay gives buyers 30 days to return items which are not fit for purpose.
You will have to pay return postage to get the ring back.
You received £15 for the ring, the other £1.32 was the Ebay Buyer Protection Fee.
I would suggest you accept the return, issue a label and get the ring back.
When you refund, you will have to refund the £15 you received plus the postage the buyer paid you. The BPF will be refunded by Ebay.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)