A madstone, known as bezoar stones, resembles a rock or marble and are concretions of mineral salts combined with hair or fibers. Most are smooth and rounded ranging in size from a marble to an orange. They're occasionally found in the stomachs of livestock as well as in whitetail deer but those from the deer were deemed more potent. Madstones form from calcium deposits similar to the way an oyster forms a pearl. Calcium clings to a foreign material then more calcium is added in layers. Cut through one and you'll find concentric rings like those in a tree. Old timers in the 1800s believed madstones held healing powers for rabies, snakebites, spider bites, and the like. The stone was placed in warm milk and then applied to the wound. If it didn't adhere it was thought they didn't have the virus. To do any good it had to stick and begin drawing out the poison. After it had drawn it's limit it dropped off and was again bathed in warm milk. The milk supposedly turned green from the madstone releasing the poison and at that point, it could be used again. They were considered highly valuable.
I have 1 I found in the stomach of a deer I killed some 20 years back. Mine is whitish, round like a marble, and between a quarter and half dollar size.