I've been sharpening knives for a long time and have amassed a good collection of synthetic stones, diamond hones and ceramics and even used the scary sharp technique with wet and dry and films. However my experience with natural stones has been limited to a couple of small pieces of Arkansas from an old Buck kit.
Embarking on a straight razor adventure I have purchased some large Arkansas stones but have yet to prepare them.
While researching natural stones I tried to make some sense of the difference between Arkansas, Jnat and coticules.
Part of that reading was the chemistry/geology of those stones. I was struck by the similarities between some of the well known naturals (SiO2 in a Sericite matrix) and Zebra Rock from the East Kimberley region of Western Australia.
Of course chemical composition doesn't define what makes a good whetstone.
I happen to have a good friend who made a tree change (Professor of Biochemistry) a few years ago and bought the Zebra Rock Gallery in Kununarra WA which is why I thought to check on his offerings.
Zebra Rock is sedimentary siltstone as is Arkansas and Jnat stone. Zebra Rock is 600-700 million years old but a variation of that is known as Primordial Rock and that is dated around 1200 million years old.
In contrast Jnat is 70 million years old and Arkansas is 350 million years old.
It is found only in the East Kimberley region near the famous Argyle diamond mine. Very few mines now operate because large swathes of that region were inundated when the Ord River was dammed creating the huge Lake Argyle.
The stones as far as I know have only been used for decorative purposes but I prevailed upon Bruce to cut me a few slabs to try as whetstones.
There are over 50 colour variations recorded and ones shown above are grey and brown/cream. The second image is the underside of the brown stone.
There is no grading system in place and no previous reported use as a whetstone and many of the names are just for marketing purposes so I had no idea how they were going to perform. The only other fact that seemed relevant was a report that primordial has a grit range of 14000 or higher.
As mentioned in another thread the grey arrived broken and I have yet to fix that but I have tried the brown/cream as is with no lapping or other preparation.
I took some kitchen knives that have previously been sharpened using a ceramic stone which left the edge with obvious striations and proceeded to slowly hone. A nice slurry developed over some time and I ended up with a more than adequately sharp knife. Examination of the edge under a jewellers loupe showed a highly polished finish, much much better than I have achieved with 2000 grit W&D.
Not only that, the feel of the steel on the hone was unlike anything I haver experienced using any of my previous stones. Keep in mind that I have never seen a Jnat or coticule much less handled one so I have no comparison base but if they perform in any way like this I understand the passion.
There are members here who are not only passionate but have a tremendous knowledge base and who would be much better qualified to assess the qualities of this stone.
If anyone would like to review the brown/cream one I will send it to them. I'm not sure how I will choose if lots of you want it but we'll work something out.
For those that are interested, here is where I got it from and my disclaimer; I have absolutely no financial interest in this.
http://www.zebrarock.biz/default.html