If the stone is faceted, there isn't any way I know of to polish a
scratch from a facet with a flex shaft without rounding the facet
junctions. Cabochons, however, are another story. This is the same
setup I use for polishing carvings.
First, there are two types of cup brushes available for your flex
shaft. Basically, they are permanently mounted on a mandrel with
bristles that face down and slightly out from the tip of the mandrel
in a sort of cup shape. White bristles are stiffer than black
bristles. I have a few of both types for each of several grits of
diamond paste I get from Graves Company. These diamond pastes come in
syringes that you use to squeeze a drop or two out at a time. There
are also diamond sprays, but I've never used them.
Load the cup brush with your chosen grit They're available from 240
(I think) to 100,000. For very light scratches, I'd probably start at
600 and work my way through the finest, working the brush evenly
around the entire stone using moderate speed with the flex shaft.
This is so I can get the same final polish on the whole stone.
Keep the brushes with the same syringe of paste (it's a greasy
paste) and always use the same brushes with the same grits, and clean
the stone thoroughly with degreaser between each grit to avoid
Some stones will final polish nicely with 50,000 grit, others will
require 100,000. Still others will polish best with a powdered oxide
such as cerium, tin, aluminum, Linde A, etc. Some of those oxides
will work better on leather, some will work better on felt and none
will work with the cup brushes. It mostly depends on the stone. Many
soft stones will polish best with Fabulustre or Holy Cow on a muslin
buff, which also puts a good polish on sterling. Sometimes lapidary
can be as complicated as gemology.
If you have a facet larger than the cup brush, you may successfully
polish out the scratch without rounding the facet junctions (i.e., a
table facet), but chances are the facet will be visibly concave. A
link to Graves' diamond paste compunds:
http://www.gravescompany.com/polishin.htm Your supplier of flex shaft
attachments will have the cup brushes.
I'm not affiliated with Graves, but the coralnut is - perhaps he can
tell you more.
James S. Duncan, G.G.
James in SoFL