Circular millstones ö Basalt ö 1500 BCE ö this type of milling instrument, with one circular stone set on another, represented an improvement on the quern and rubbing stone because grain could be poured onto the grinding surface through the space near the central pivot at the same time that the upper stone was being rotated to produce flour. The ground flour escaped from the sides and fell onto a piece of cloth or basketry that was placed under the millstones.