A millstone is usually made from millstone grit - and there is plenty of it about - but this stone isn't it. Millstones that have been discarded usually have both wear patterns and radial grooves (rather than a rude spiral). There has been a mill within walking distance, a few hundred years ago, but it was a woollen mill - and half a mile away. It's a pity the stone is fragmented, and mostly lost, but this is the only carved spiral in this part of the country (NW England). Calling it something it isn't won't help in the slightest. Where is the nearest next one to this ? Ireland, Scotland or Wales ?