A bezoar is a sort of calculus or concretion, a stone found in the intestines of mostly ruminant animals. There are several varieties of bezoar, some of which have inorganic constituents and others organic.

Bezoars were formerly sought after because they were believed to have the power of a universal antidote against any poison. It was believed that a drinking glass which contained a bezoar set within would neutralize any poison poured into the glass. The word "bezoar" ultimately comes from the Persian pâdzahr (پادزهر), which literally means "protection from poison."

A famous case in the common law of England announced the rule of caveat emptor, "let the buyer beware" if the goods he purchased are in fact genuine and effective, in a case over a purchaser who sued for the return of the purchase price of an allegedly fraudulent bezoar. (How the plaintiff discovered that the bezoar did not work is unfortunately not discussed in the report.) Judicial scepticism over the alleged magical powers of bezoars may well have justified this judgment in this particular case. The ruling, however, was seized on and formed an impediment to the formation of effective consumer protection remedies and the law of implied warranty well into the nineteenth century.

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