A shriveled, kidneylike “stone”that comes from the stomach of a goat, protects from most poisons
- Professor Snape quizzed Harry on the bezoar during the very first Potions Class.
- In his fourth year, while distracted by thoughts of Cho Chang and the Yule Ball, Harry forgot to add a bezoar during a potions test and received low marks.
- In the Half-Blood Prince’s Potion Book, Harry finds the notation to “Just shove a bezoar down their throats” as an antidote. Harry decided to show Professor Slughorn a bezoar instead of mixing a complicated antidote and won ten points for Gryffindor, for which Hermione accused him of cheating.
- On Ron’s 17th birthday he is poisoned by some mead that was meant for Dumbledore, and Harry saves his life with a bezoar.
A bezoar seems to be essential in creating antidotes to poison.
From Arabic "bazahr" - originally from the Persian "pad-zahr" - "pad" meaning "protect or guard" and "zahr" meaning poison
Actually a bezoar isn't a stone per se, but a hairball that looks something like a stone; in legend bezoars are indeed supposed to have the properties attributed to them by JKR.