Nanette,
Glad your scratch wasn’t a deal breaker. the big three of polishing
are cerium oxide (glass, quartz, opal, other soft stones), alumina,
and 50k diamond. Actually, 14K diamond on cabs works pretty well. A
felt wheel on the foredom and one or the other of these polishes
should work. A little water or oil for lube. The list of what works
with what would be long, but ought to be pretty easy to find
somewhere on the 'Net. Either the felt buff or a little leather buff
ought to be all you need. If a scratch is deep and won’t come off
with a repolish, you might have to prepolish with 600 or 1500 or 3K
diamond grit. Cabochons are not that hard to polish and you could
easily try one thing after the other until something worked. The
polishes that don’t work will not damage the stone.
Hope this is somewhat helpful as a general guide.