Bezoars are formed when undigestable material accretes ith the stomach of a human or animal. there are several different kinds, classified by the type of material that forms the bulk of the mass. Bezoars can be formed of hair, plant fibres, and even the gel capsules that medication is delivered in. Trichobezoars, pictured here:
are the hair-based type. They often build up in the stomachs of young girls who chew on the ends of their hair. Trichobezoars are packed with partially-digested food, and therefore smell terrible. As you can see, the bezoar conforms precisely to the shape of the stomach, and usually must be surgically removed.
Bezoars have a long mythic history - they were traditionally believed to be proof against poison, and European royalty frequently had them set in gold, like gemstones.
Bezoars are incredibly gross.