Use this stone to put the final, polished edge on the blade.
Each Natural Toishi is unique. Natural stones are cut from native fine-grained mud stone and are not graded. Some areas may be slightly coarser than others on a single stone, so it takes time to understand and use them well. But with a little practice these fine grit stones will put an extremely sharp edge on the blade.
Because they are cut out of the earth, some stones may have chipped corners or may not be perfectly flat or rectangular. This does not effect their usefulness at all and is not considered a flaw. If you require a perfectly shaped stone, you might consider a ceramic stone instead.
Natural stones create their own creamy paste very readily, so using a Nagura Stone is not necessary.