One of the key stones mentioned in Tsukiyamateizōden (zenpen). According to that work, it is placed on the shore. It is thought to have been shaped like a stone used for grinding ink, and to have been arranged as a set with the ink brush stone and brush rest stone.
Japanese Garden Dictionary: A Glossary for Japanese Gardens and Their History
(C) 2010 by Ono Kenkichi and Walter Edwards. All rights reserved.