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.
Name:  Naniwa6k.jpg
Views: 118
Size:  22.4 KBName:  Naniwa8k.jpg
Views: 113
Size:  29.8 KB