For novelty, I'll sharpen the iron with this stone freehand before I give it the test setup of honing guide with back bevel. I think technically the stone probably cuts all of the particles except the volcanic ash (natural aluminum oxide) probably just rubs the vanadium a little and won't fully refine the edge like diamonds will.
Are you OK with me grinding off the back bevel that's already there? I will grind with CBN so that there isn't any significant heat. Flattening that out to restart would be quite a bit of work vs. just grinding it perpendicular and then resetting the bevel.
As far as the result of "less than perfect" sharpening with natural stones, I get a very good working edge on almost everything so far if I have a natural stone except for the SGPS and A2. the SGPS seems to hate almost everything (but loves the deburring wheel followed by a charged buff), even some substrates, and the A2 really hates the washita.
Though nobody's asking, I have probably bought 150 stones from japan and resold most of them (not really making any money, just giving people a relatively cheap source for a graded stone). I find sellers who list less than full size and length stones (sometimes they're full size), but who sharpen something on the stone. If a stone like this comes up (it's a 1kg stone, not tiny by any means, just not full length - it's still a lifetime stone for someone who uses it with care) and it shows a pure black heavy coloration on the surface, you can almost guarantee it will be a very strong cutter. Especially if the seller shows the result of cutting on various stones - you can see which are strong, and which might be less strong or softer by the slurry on the surface.
I hate to say it, but this has been even more reliable than some of the mid-cost dealers that I've dealt with who are selling only new stones. I do have stones that were up to about $800, but faster cutting slightly less fine stones like this (I'd guess 10k grit equivalent on synthetics) you can use without being worried what will happen with them and they are the type where you can sit down, cut rows of half blinds and keep a japanese chisel sharp with one stone indefinitely. A real treat.
My favorite razor stone so far is a bench stone that was flipped by an antiques dealer. It had a weak very old stamp on the end and ended on auction for $35. Not all of the inexpensive ones are good, but the cost of the used stones makes it not much of a risk.
The feel of a stone like the one in the picture is something that synthetics can't duplicate. The only thing that comes close is a frictionite (which isn't really that fine cutting of a stone and the bench stones (#825) are very expensive if you can find one). They do have a superbly dreamy feel, just like a suita, but are not really suited for bench work.