Arato-ishi
Oomura-do Arato-ishi; a very coarse stone for when the Togishi needs to execute significant groundwork; heavy repair, rust removal, and setting the basic geometry that will be shaped further on Kongo-do, Binsui-do and Kaisei-do.
Oomura, or Omura, stone seems to be in either limited supply or suffering from limited demand; I’m not sure which. They are a type of sandstone, which is fairly aggressive with good cutting power. They absorb a good amount of water, so I’m thinking that sealing the sides would be a very good idea.
For honing striaght razors, the Omura is a bit harsh but still excellent for removing big chips on heavy blades. It’s a remarkable option for working on heavier blades, cutlery and tools also. To draw a parallel, one might look at this stone as the natural equivalent of a 200-400x synthetic.
This example was sent to me with an Omura Tomo Nagura, which is a very unusual and extremely useful addition. The Tomo is huge, and with slightly different coloring but, in use, it’s a perfect match.
Approx. Dimensions: 8.9 x 2.75 x 2.25 / 226 x 70 x 57 mm
Weight: 74 oz / 2090 g