|Pliny the Elder, in his Natural History, mentions the Druids manufacturing an object called the ovum anguinum, or "snake’s egg," which in Gaulish would have been called ouion natracos. In British and Irish tradition it was known as the snake’s gem, or Druid’s gem.
The Druid’s gem was a round bead about one to one and a half inches in diameter, decorated with spirals or swirls. It was sometimes made out of ceramic (in Scotland), sometimes glass (reported in Wales, Scotland and Cornwall) and, as in ancient Gaul, sometimes out of empty whelk egg cases or despined sea urchin shells. According to Pliny, a Gaulish man who was also a Roman citizen wore an ovum anguinum to court for good luck, but lost his case because the Roman magistrate was prejudiced against him for wearing a Celtic charm.
The snake which dwells in the Underworld is said to have been the origin of these charms. Mythically, Underworld snakes are always guardians of sacred treasures (such as the salmon in Irish tradition, Fafnir in the Volsunga Saga, and the snake in the garden of Eden). The snake as guardian of an Underworld mystery and/or treasure is a theme which goes back to a source common to both Indo-Europeans and Semites.