Naniwa Diamond stones, 6k and 8k. $200
I don't think these are really for razors. I tried various mediums, but the edge tends to get nicked. However, under running water the edge did not nick.
I also have not tried these with extra hard steel.
I have used the 6K with kitchen knives, and that's rather fun.
The 8K doesn't seem to be available anymore, at least with a simple google search.
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