This is the heart of the milling operation.
Millstones work in pairs, a stationary bed-stone, and a rotating runner stone above it.
This example is a runner stone. Note the black bar across the centre. This is called the rynd and it supports the weight of the runner stone.
The aperture through the runner stone is called the eye of the stone.
Grain which is fed into the eye of the runner stone is then reduced to meal in the minute gap between the stones.
The pattern of grooves cut into the stones distributes the grain, break it down into meal, and help it to move towards the outer edge where it is discharged. This pattern of grooves is called the dressing of the stone, and as a stone wears it must be re-dressed in order to keep the sharp edges.