noun
whetstone; grindstone; sharpening stone
A stone used for sharpening blades such as knives, chisels, or swords. Both natural and synthetic stones are called 砥石.
包丁を砥石で研ぐ。
I sharpen the kitchen knife on a whetstone.
この砥石は目の粗さが違う面がある。
This whetstone has sides with different grits.
Compound of 砥 (to, 'whetstone' or 'grind') and 石 (ishi, 'stone'), with rendaku voicing to いし → といし. The kanji 砥 is a phono-semantic compound with 石 (stone) and 氐 (phonetic component).