Bezoar

As every First year Potions student learns, a bezoar is a stonelike mass taken from the stomach of a goat that acts as an antidote to most poisons, but does not work on everything. This object is usually made of hair, plant fiber, or similar indigestible matter that stays in the gut of an animal and forms a hard ball or "stone". {| class="toc" id="toc"
 * "A bezoar is a stone taken from the stomach of a goat and it will save you from most poisons."
 * —Snape lecturing in Harry Potter's first year Potion class. [src]

Contents
1 History
 * 2 Known Users
 * 3 Etymology
 * 4 Appearances
 * }

History
There is a small stock of bezoars in the Potions Classroom store cupboard. Severus Snape asks Harry Potter where to find a bezoar in Harry's first Potions class; when Harry is unable to answer this or any of the teacher's other questions, he earns Snape's contempt. Three years later, in a Potions exam, Harry forgets to add a bezoar to his potion, because he was thinking about asking Cho Chang to the Yule Ball. This earns him bottom marks.

As a student at Hogwarts, the Half-Blood Prince wrote "Just shove a bezoar down their throats." across the list of antidotes in his copy of Advanced Potion-Making. During his sixth-year class about antidotes, Harry, who was unable to understand Golpalott's Third Law, fetched a bezoar from the cupboard and showed it to Professor Horace Slughorn as the answer to the poison he was supposed to be analysing. This gained Harry Slughorn's admiration and ten points to Gryffindor for "sheer cheek."

Known Users
On 1 March, 1997, Ron Weasley was poisoned when he drank some oak-matured mead that was intended for Professor Dumbledore. Harry, remembering the above mentioned incident in Potions class, quickly found the bezoar he had given Professor Slughorn and shoved it down Ron's throat, saving his life.

Etymology
Bezoars are undigested clumps of matter that accummulate inside a digestive system, similar to a cat's hairballs. They are used in Chinese herbology, and are claimed to remove toxins from the body. The word "bezoar" may come from the Persian pâdzahr (پادزهر), which literally means "protection from poison."

Appearances

 * Harry Potter and the Philosopher's Stone (First mentioned)
 * Harry Potter and the Philosopher's Stone (film) (Mentioned only)
 * Harry Potter and the Philosopher's Stone (video game) (First appearance) (GBC version only)
 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Half-Blood Prince (video game)

Thank you very much to the Harry Potter wiki from which I have gotten all my material!