Every household in the days of the apostle needed a millstone to grind grain into flour (from which in turn bread was made). A flat stone was moved over a stone surface with grain in between in order to separate the kernels of grain from the husks. This was typically the work of a slave (Exodus 11:5), though if the household did not have a slave, the mother and/or daughters did this kitchen work (Matthew 24:41). Clearly, a bigger stone could grind more grain in quicker time. But bigger stones required more strength, and so larger households used donkeys to operate the mill. In that case, the millstone was set to operate around an axle. This is the sort of millstone referenced in the present verse. As a “great millstone” is too much for a person to operate, so also is it too heavy for a person to lift (let alone throw). That great weight represents Babylon. But this “one mighty angel” can do the task.
21 Then a mighty angel took up a stone like a great millstone and threw it into the sea, saying, “So will Babylon the great city be thrown down with violence, and will be found no more;