I’m not the expert most of the fellows here are, but I’ve never had a knife that was too hard for the diamond stones. Your diamond stones will even abrade ceramic blades. I have, however, had instances of knives that simply wouldn’t cooperate. In each case, I found that the knife was moving around on me in the vise. Check for movement by twisting on the knife’s handle while it’s in the vise. If it rocks from side to side, you’ll never get it sharp.
If you find that this is the case, try clamping the blade in the vise wrapped with a patch of tape or leather (usually chamois). I use a heavy duty paper towel.
That said, you could theoretically run across a knife here and there which had not been tempered. Tempering is a process to reduce the hardness in a blade after it has been hardened. Usually, it’s done by cooking the steel in an oven at a relatively low temperature for a few hours. It reduces brittleness and internal stresses in the steel.
Some say that stainless is more difficult to bring to a very fine edge than carbon steel, but I haven’t noticed that stainless is any more difficult to grind than carbon steels. At least not with the diamond stones.
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