The type of stone most suitable for making millstones is a siliceous rock called buhrstone (or burrstone), an open-textured, porous but tough, fine-grained sandstone, or a silicified, fossiliferous limestone. In some sandstones, the cement is calcareous.
Millstones used in Britain were commonly of two types:
The surface of a millstone is divided by deep grooves called furrows into separate flat areas called lands. Spreading away from the furrows are smaller grooves called feathering or cracking. The Marshalls and lands are arranged in repeating patterns called harps. A typical millstone will have six, eight or ten harps. The grooves provide a cutting edge and help to channel the ground flour out from the stones. When in regular use stones need to be dressed periodically, that is, re-cut to keep the cutting surfaces sharp.
Millstones come in pairs. The base or bedstone is stationary. Above the bedstone is the turning runner stone which actually does the grinding. The runner stone is supported by a cross-shaped metal mill rynd fixed to a "mace head" topping the main shaft or spindle leading to the driving mechanism of the mill (either water or wind powered). The pattern of harps is repeated on the face of each stone, when they are laid face to face the patterns mesh in a kind of "scissoring" motion creating the cutting or grinding function of the stones.
Millstones need to be evenly balanced, and achieving the correct separation of the stones is crucial to producing good quality flour. The experienced miller will be able to adjust their separation very accurately and precisely.
A Millstone around one's neck is also a phrase used constituting a metaphor meaning a burden or large inconvenience one has to endure.