I just haven't had enough idwalls to compare, but my two - and now I have one, i sold the smaller and finer one that I have to a shaver.....give the impression that they're cutting more finely than a trans ark. But it could be, if i infer from your mention of impurities, they just aren't cutting as fast for reasons that have nothing to do with relative fineness.
I always considered a charn "an arkansas with a soft temper", poor way to put it, but you can feel novaculite vs. slates, especially under oil. There is more slurry dulling and less of a filing feel if you scuff a novaculite stone. The first time scratched a charnley, i was surprised by it. I was also surprised by the variability.
I thought they were wonderful looking stones, though, and sometimes that's misleading.
One of the best cutting japanese stones I ever had was a plain green "narutaki tomae" stone coming from a hardware store in japan. I can't remember why I bought it, but to my surprise, it was an aggressive cutter, better than the ubiquitous imanishi ohira tomae, that are both a little coarse for what they should be *and* a little slow relative to a better stone.
The second green stone that I had was probably either mauroyama or narutaki or nakayama, but it was too old to be labeled anything other than "original stone" with a very old stamp. I found it on buyee for $32 and it was the most aggressive fine stone (as in, not by feel fine, but proven fine under the microscope) I've ever had. I figured it looked old and I would use it for razors because it was picked by a barbershop antique picker who listed it as unlabeled. I cracked off a layer of glue or something that was along one side and it had a faint label underneath.
I rarely marked up stones, but I did mark that one at $275 and listed it on ebay years later (should've kept it, but razor comment above applies - I hone a razor once a year and any fine stone is good for that) as "the best fine stone anyone will ever find, mine uncertain, but likely an original mine". the buyer was astounded by it - so good enough. I lost that much gain on more than one other stones. It would've been $1000-1500 from any other seller. It, too was kind of boring looking (aside from not being the common lighter atagoyama green, and a deeper more intense green that you see in some of the better earlier stones).
I had a lot of other very interesting looking stones that were average or slightly below average users and got suckered into buying a large number of coticules trying to find one that was both pretty and worked as well as it looked.
That aside, I wish I'd have weighed the cretan that I have before introducing it to oil. Mine has done so much tool sharpening that it looks jet black, so I didn't bother taking a picture of it to compare to others shown above.
(in keeping with your link - there must be novaculite of good quality all over the world.
when I mentioned to a pair of folks into a combination of geology and chemistry that novaculite is a better cutter than most cherts, they said "there's no way, they're both cryptocrystalline quartz".
things like that have taught me the value of outcomes rather than substituting perceived outcomes based on characteristics).