Throw it in the dishwasher. Just kidding. You will have to polish it by hand with a soft cloth and metal polish. It will take time and may not com out perfect but will look better. I had to do the same to the jeweled bolt of my Savage 99.
I had the same problem on the Model 12 16 gauge I redid awhile back. After I completely disassembled the gun and the breach bolt, I cleaned it up with Flitz. It took a while, but I just kept polishing while I watched TV. It cleaned up nice. If you’re not the patient type, you can use a Dremel and a polishing wheel with the Flitz, just make sure you go slow. (I have used the dremel on an OLD nickel plated Iver Johnson revolver. It worked pretty well.)