I bought a Hi Tech Diamond lapidary polishing machine with numerous grade diamond wheels for just that purpose, cleaning and restoring odd sized crystals. It's very much a time consuming endeavor, and I've found that the vast majority of crystals are easily replaced with generic mineral glass. If you have a crystal that is scratched, and you can feel the scratched area with your fingernail, then it is going to take quite a while, and a number of different wheels, plus cerium oxide, to get it back to perfect. What I'm saying is that it just wouldn't be worth the time in my estimation, unless you were doing the work yourself.
Aloha