noun
A stone used for sharpening blades or grinding tools. Both whetstone and grindstone are covered; context determines the specific type.
包丁を磨石で研ぐ。
I sharpen the kitchen knife with a whetstone.
職人は磨石を使って刃物の切れ味を戻した。
The craftsman restored the blade's sharpness using a grindstone.