Fossils of a sea urchin (Cypeus ploti) were abundant in rural England. Since they were unusually regular in size and mass, Oxfordshire milk-maids used them as a weight for the butter scales; this practice continued into the 18th century. The fossils were known colloquially as "Chedworth Buns," "Checkbury Buns," or "Poundstones."
"Stone" is also a common British weight measurement. Officially, it is now 14 pounds or 1/8 of a hundredweight. Historically, it evolved from a practice similar to that of the rural milkmaids:
Originally any good-sized rock chosen as a local standard, the stone came to be widely used as a unit of weight in trade, its value fluctuating with the commodity and region. In the 14th century England’s exportation of raw wool to Florence necessitated a fixed standard. In 1389 a royal statute fixed the stone of wool at 14 pounds and the sack of wool at 26 stones. Trade stones of variant weights persist, such as the glass stone of 5 pounds. The stone is still commonly used in Britain to designate the weights of people and large animals. --Britannica