medium-grade millstone
Refers to a millstone of intermediate quality, used in traditional milling contexts. The term is relatively rare and may appear mainly in historical or technical discussions.
Documents from the Edo period record medium-grade millstones called 青砥.
The term combines 青 (blue/green, but here possibly indicating a grade or type) and 砥 (whetstone/millstone). The exact origin of the grading term is uncertain.