Iyoto (Iyo whetstone) is said to be the oldest whetstone in Japan.
This Nagura whetstone that was mined from Tobe-cho, Iyo-gun, Ehime Prefecture,
and was mined until around 1965, and was often shipped all over Japan.
There is no document that Iyoto was used as a whetstone for kitchen knives,
but It is so important that this grindstone is always used for ground polishing of
Japanese swords.