Mikawa Shiro Nagura Chu-Nagura 4.6oz. (132g) Japanese Natural Sharpening Stone
Approximately 39mmx42mmx35mm
According to Dr. Kuniichi Tawara who is a Metallurgy doctor, and very famous Japanese sword researcher, Koma molecule is 10 to 20 micron. It is Atsu (Chu-nagura). It should be good for one stone before starting the fine grit sharpening.
For razor sharpening, it is used one or two before the process sharpening by fine grit whetstone like Nakayama, Ozuku, Wakasa, etc.