Borax, in early modern folklore, was sometimes identified with the “crapaudine”, a supposed stone said to be found in the heads of toads. This belief was widespread in medieval and Renaissance lapidary literature, where the stone was often credited with apotropaic and medicinal properties, such as neutralizing poisons, inducing sleep, or protecting the bearer from illness. The crapaudine was typically described as small, dark, and smooth, and was sometimes embedded into rings or amulets, particularly among those fearful of poisoning—an ever-present concern in aristocratic and courtly settings.
Naturalists and physicians in the 16th and 17th centuries debated the authenticity of the crapaudine. Some, like the French surgeon Ambroise Paré, considered it a superstition and attempted to debunk it in his writings. Others, influenced by Aristotelian or alchemical frameworks, theorized that such stones could form naturally inside animals due to environmental or physiological factors, much like bezoars found in goats or deer. However, modern zoological science has confirmed that no true mineral or crystalline object is formed inside the heads of toads, and many historical specimens were either plant galls, calcified masses, or outright forgeries.
The confusion may have also arisen due to the overlap in terminology. While “borax” today refers to sodium borate, a naturally occurring mineral used in cleaning and metallurgy, older texts occasionally used the word to refer to mysterious mineral substances or alchemical compounds, contributing to the conflation with the crapaudine. The crapaudine continued to appear in herbals, lapidaries, and cabinets of curiosity well into the 18th century, often listed among other marvels like the unicorn horn or the basilisk’s remains.
Such beliefs were part of a broader medieval and early modern tradition in which certain animals—especially reptiles and amphibians—were believed to harbor magical stones. Similar stories include the serpent’s “ophite” stone, the toadstone or bufonite, and the salamander’s association with fire. These legends were reinforced by bestiaries, which combined natural observation with allegorical and moral interpretations.
