Here's an example of half of a mill-wheel in a coat of arms:
I offer this to underline Chris's point that what is shown in the original image is not a mill-stone, as seems to be intended. It could be a mill-wheel, the thing, usually made of wood, that is pushed by the water to drive an axle that then turns the mill-stone. A millstone would be simply a large disk with a square hole in the center. Either of these, or the mill-rind mentioned by Chris, could be an appropriate symbolism of the trade of a miller or of a person named Miller (or Muller, Meunier, Molinero, Molinari, and so on).
(Personal note: The arms are those of the Mohler family of Diegten, Basel-Land Canton, Switzerland, and therefore canting, as Mohler means miller. I found them because they belong to my 3xgreat-grandmother's family, although they weren't assumed by her very very distant cousins until 1964, nearly 300 years after her 3xgreat-grandfather left Diegten.)