Alma, A lot depends on the size of the stone, how it is set and where
the scratch is on the surface. For instance, if the scratch is on the
crown, easily accessible and no other part of the setting or other
stones would interfere, such a scratch could be easily removed with a
Crystal Pad and diamond. I would start with 600 grit to see if it did
the job. If not go back to 280 grit, then 600, 1200 14,000 and finish
with a cerium oxide on felt at slow speed. This is pretty standard
for any chalcedony/quartz/agate/jasper material. The Crystal Pads can
be purchased from http://www.ganoksin.com/gnkurl/qw
(Used to work for them part time and a happy customer). Ask Pete for
the small wheels usually used in his carving kits.
Hope this helps, cheers, Don in SOFL.