I've got some metal ones (no composition given) that are simply listed as being suitable for stainless steel frets. Obviously a composition that's harder that stainless. I think stainless tends to pull the diamond 'dust' off the file surface, so it becomes less and less effective. I know that even the standard cheap Chinese orange-handled crowning file that always comes up on Amazon and eBay when you look for crowning files, was a much much better file on stainless than my diamond files were.

I know that the description for diamond files says 'suitable for stainless steel and normal frets', but my own experience, and quite a few luthier's experiences, is that they aren't as good and you'll spend a lot longer reshaping the frets.