Fossils used to weigh butter

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."

Image credit here (it doesn't look much like an urchin because fossilisation does not preserve the spines).