Have you tried optical grade cerium oxide also known as French
cerium oxide? What are you using to polish with? Try cerium oxide on
split cowhide. If that doesn’t work try tin oxide on split cowhide.
Try both on a moist muslin buff. Try mixing the cerium oxide with
petroleum jelly and buff with a muslin buff. Make a wooden wheel that
will fit on a tapered shaft and try it with cerium oxide, tin oxide,
or diamond paste. When I have a stone that just won’t take a polish I
just keep trying different things or combination of things.
Silversmith and Lapidary Artisan
Rocky Mountain Wonders
Colorado Springs, Colorado