grindstone
grind.stone \'gri-n-.sto-n\ n 1: MILLSTONE 2: a flat circular stone of natural sandstone that revolves on an axle and is used for grinding, shaping, or smoothing
Webster's English Dictionarygrind.stone \'gri-n-.sto-n\ n 1: MILLSTONE 2: a flat circular stone of natural sandstone that revolves on an axle and is used for grinding, shaping, or smoothing