When polishing a crystal I first use several grades of wet/dry emery cloth like 400 then 600, then 1500. After that I'll buff the crystal using my Dremel tool and soft felt wheels. I use Fabuluster compound and the result is a crystal that looks like new. The important thing is to keep the tool moving around, don't force it and let the compound do the work. Don't stay in one spot too long or it will get hot and melt the crystal.
You can also find the right wheels for bench grinders. The bench grinder needs to be speed controlled so you can slow it down enough for use on plastics like crystals.