What did Erik give Christine for protection?

From the text:

"Oh, so Erik gave you that ring!"

"You know he did, Raoul! But what you don't know is that, when he gave it to me, he said, 'I give you back your liberty, Christine, on condition that this ring is always on your finger. As long as you keep it, you will be protected against all danger and Erik will remain your friend. But woe to you if you ever part with it, for Erik will have his revenge!' ... My dear, my dear, the ring is gone! ... Woe to us both!"