The best person I knew is now departed (RIP Leroy Johnson) and I really don't know who, if anybody, does it now. The thing you have to worry about is that if the damage is too deep grinding it down not only removes the damage, it removes or alters the pattern or other elements such that the marble is no longer worth having. This is particularly true with the old Germans - you can lose the outer decorations if you're not selective about which marbles you try to polish. Or you might wind up with a marble that retains it's interesting elements and has a shine on it, but still has divots where the damage was originally.