You can try soaking in water-soluble C 100, that is, drink, or take Vc yourself, which is mild in reducibility. I used to have a silver ring soaked in potassium permanganate and it was all black. Soak on that day. If you want to keep it, put it in a box and in a desiccant (such as chewing gum). It's no problem to soak it in oil, but I feel sorry for that ring.