the.com/grinding stone

means A hard, flat or rotating stone used to sharpen, crush, or grind other materials — from blades to grain to pigments.

from A plain compound of 'grind' (from Old English 'grindan', meaning to crush or rub into small pieces) and 'stone' (Old English 'stan'). No hidden trick to it — the name simply describes the oldest trick there was: rub something hard against something soft, over and over, until the world changes shape. Querns and millstones using this same principle are among humanity's oldest surviving tools.