OPINION: ENGAGEMENT RING FROM EX?

I have my engagement ring and my wedding ring from my ex-husband. The engagement diamonds that were his grandmothers, but he did not want the engagement ring back. I plan on keeping the wedding ring, as he is keeping his and we are friends. What the heck do I do with the engagement ring? Do people buy them second hand? I also feel bad about it? I have moved on and am in a wonderful relationship w someone amazing. I think it’s the part about the grandmother’s diamonds that stresses me out.