Is blackmailing my ex girlriend a sure way to get my ring back. i desperately need this ring back?

That's horrible. If you blackmail her it will come bite you in the butt later. It won't make her look half as bad as it will make you look. You'll be the psycho who blackmailed her. They can file a restraining order, and things can get really ugly, fast. You gave her your moms ring, and that was your decision. She will get hers some day but it's not up to you to give it to her. I know you're mad and you have a right to be. But don't be that guy.