The Tsushima Nagura is unique in it's own way for a few reasons, let me tell you why. Firstly, this stone was quarried from an underwater mine in 1983. It has been used quite often by traditional wood workers for sharpening their tools (as well as many other natural stones).
This Nagura acts in a similar form to a Tenjyou 天上 or a Mejiro 目白which I assume will raise the question, what is the purpose of using one when you already have a Mikawa nagura set?
I have used the slurry from the Tsushima nagura on my hard Narutaki toishi. First off, here is where it lacks some goodness, the colour of the slurry is light grey which can be deceiving with swarf and it is slower than Mikawa naguras so it will take more time to cut when you're only using it for you mid-work on a razor, but why would you when it's capable of so much more?
Here is where it shines, the slurry from this stone does NOT stop breaking down and keeps getting finer and finer to the point where you could easily finish on it, opposed to a Mikawa nagura where it will cut faster, break down quicker and the slurry will also dull quicker forcing you to use your next nagura in your progression, not to say it's a bad thing at all but Mikawa naguras work better in a set. The Tsushima slurry just lasts and can fit in many different progressions, for me, it will work straight after the Botan ボタン and once the slurry has been broken it can replace the Tenjyou, Mejiro and Koma, even the Tomo nagura depending on the level of abrasion and grit fitness of your tomo.
On softer steels I have successfully used it straight after the bevel set for a one stone hone, in this case the Chosera 1k and was able to finish on a harder base stone with the slurry from the Tsushima which resulted with a HHT 3-4 prior to stropping, and a solid 4 after stropping on linen and leather. What I also did notice that it left a very high polish on the bevel, comparable to my 10k Chosera which I found to be quite odd for a Japanese natural stone as they generally leave a hazy sandblasted look to the finish, another indication of the slurry getting very fine. Refer to the photos below.
Overall the Tsushima works as a great pre-finisher or a finisher and provides great feedback, although many do claim that they're quite rare, I don't find that to be the case. They're priced quite well and are quite uniform, consistent, clean and generally free of toxic lines or inclusions. You may contact me if you're on the search for one or even the full size Tsushima and I'll do my best to assist you. Hope this helps. Mark
Photo of Tsushima Nagura finish at 400x
Standard photo of the bevel