where I got this from

The Bezoar is a red stone, which is supposed to be precious and possess magical properties. It is supposedly found in bodies of certain animals (in Harry Potter Prof. Snape tells the class it is found in the stomach of a goat, which does tie in with the fact that there is a type of goat called bezoar). These stones used to fetch ten times their weight in gold, as they were used as a remedy against poison and disease. For this purpose they were taken both internally and worn about the neck. It is said that there are nine varieties of bezoar, differing greatly in composition. These may generally be divided into those that consist mainly of minerals (stinky stuff) and those that consist of organic matter (ishhy stuff).

Early naturalists thought that when oriental stags were getting on a bit they fed on serpents, which renewed their youth. In order to counteract the poison that they had just eaten, they plunged into a running stream, keeping their heads above water. This caused the viscous fluid from their eyes to be distilled, which was then solidified by the heat and sun, and formed the bezoar.