Do you mean you want to ask her to renew your vows or is it that the two of you are divorced and are asking her to remarry you? Top Wedding Questions Forum Moderator - "Write your sorrows in the sand, your blessings in stone".
As long as you know that you don't need a ring then really any type of ring would do. Consider how your wife will wear this jewelry. Will she replace her wedding band? Will she wear it on the right hand? All of these are really personal decisions, but, truth be told, my favorite ring is the diamond eternity ring with diamonds all the way 'round! Top Wedding Questions Forum Moderator - "Write your sorrows in the sand, your blessings in stone".