Millstone Grit. A course sedimentary rock used to make millstones.

a hard coarse-grained siliceous sandstone

Gritstone is a sedimentary rock composed of coarse sand grains with inclusions of small stones. It is a coarser version of sandstone. It was laid down in the late (upper) Paleozoic era, in the Carboniferous period, in delta conditions.

See Grit, n., 4.