I experimented with a few things, didn't have much luck with the peroxide trick. Tried the olive oil suggestion and that didn't seem to make much of a difference.
Hot water and some Dawn dishsoap with a toothbrush and Qtips - and patience cleaned a lot of them up pretty good. I also use a tumbler with a coarse grit - let them tumble for a few days.
I experimented with Tarnex and would not recommend that at all.
The wheaties I used tarnex on were black as coal when I found them, you can see some of the black yet in this picture. The tarnex took away that nice penny look.