David Weaver
Woodcraft sells these. I've had maybe two from them and 6 elsewhere. They can vary - some self slurry, some are gritty feeling, and some are smooth on slurry and very sticky feeling on clear water (the last are the best type I've come across).

They smell slatey or chalkey. to work up a slurry on them, a fine diamond hone is perfect, or you can make a slurry stone out of something similar (see stone picture attached to this).

They are pretty easy to use with a slurry, and on clear very sticky on something like a blade back, slow and sort of unpleasant to use. On small tools like carving tools, chisels and razors this stickiness isn't an issue. I think the reason they're not more popular is that they're slow and most people who try to use them with just water probably don't like what they find. I never tried using one with soapy water, but that may make them more workable (the slurry works fine).

slurried

clear

Not a great deal of difference. The edge finish is even, but there is a little bit of evidence of slurry dulling (which makes for a smoother feel rather than harsh initial bite - the dulling isn't significant enough to make them feel "dull")

Mujingfang or woodwell distributes the stones that woodcraft sells. I have gotten them from aliexpress, too - in my estimation, the ones that come with a picture of the same box that woodcraft has are less risk, but I've heard shaving folks say that they got four from woodcraft and two good, two not that great, etc.

too slow for flat tools on clear water as is, though, and slow on slurry - usable, but nothing stands out that would make you use them over another stone like an ark stone or idwall (the idwall is far better and can sometimes be found for a similar price or a little more).