Is it possible to use a wheel with a slightly coarser grit
instead? Or if there's another tool you've had success with?
There really is only one abrasive type that's safe around most
stones, and that is pumice. The italian "silipum" silicone bonded
pumice wheels are my favorites, and are somewhat softer/more flexible
than the bluish rubber bonded ones. The silipum wheels are a grey
color for the fine grit, and greenish grey, or greyish green (what
color do YOU see?) for a medium grit. That's a relative grade, since
both are pretty fine. But the greenish ones cut slightly faster.
Both leave a mostly matte finish, but not one with visible scratches.
Just dull. There are, of course, many other abrasives you could use,
but they tend to be harder, and thus not safe for most stones. But
there are ways around this. For example, you can get little abrasive
rods, or bullet point shapes, in a variety of color coded grits. If
you run one of these against a small diamond burr to cut a small
hollow in the very end, you then could use that like a cup burr, and
if you're doing it carefully, you could fairly easily avoid touching
the stone with the abrasive, while still working the metal of the
prong. Another option is not a rubber wheel at all, but a hand held
tool. You can get little plastic handled brushes where the brush is
fiberglass, which extends just a little past the end of the handle. A
knob on the other end extends the brush as it wears. The glass brush
leaves a light satin finish on the metal, and is soft enough to be
safe with most stones.
hope that helps