1. Bezoars

In Harry Potter and the Sorcerer’s Stone, Professor Severus Snape explained that “A bezoar is a stone taken from the stomach of a goat, and it will save you from most poisons.” Ludicrous as it sounds, Snape wasn’t entirely wrong. A bezoar is kind of like a stomach pearl: An animal eats something it can’t digest, and mineral salts collect around the foreign object until a stone is formed.

During the Middle Ages, doctors prescribed bezoars as an antidote for poison. Swallowing or wearing these magical gut lozenges as jewelry was common practice among the rich and powerful, who had every reason to suspect someone was going to poison them.

Human bezoars form without the crunchy coating, but they do come in a variety of flavors: There are phytobezoars (made of indigestible plant matter like sunflower-seed shells), pharmacobezoars (made of undigested medicine), and, most commonly, trichobezoars, or hairballs.

2. Otoliths

You’ve got stones in your ear. Little tiny ones, made of calcium carbonate (better known as chalk). These little ear stones, or otoliths, help you orient yourself in space. When you turn your head, the movement of your otoliths sends a message to your brain about where you are and how fast you’re moving to keep you upright. It’s a big job for a little stone.

Most, if not all, vertebrates have otoliths, and scientists have discovered myriad uses for them in research. You can tell how old a fish is by looking at its otoliths. Space programs have sent all kinds of animals into the sky while monitoring their otoliths to see how they fare in zero gravity. (Spoiler: They don’t like it [PDF].)

3. and 4. Kidney Stone and Gallstone

Even the words are enough to make some people cringe. Kind of like their cousins the bezoars, kidney stones form when there’s too much of one substance in the kidney. The most common kind is made of excess calcium. When urine is too acidic, often as a result of eating a lot of meat or shellfish, uric acid stones can form. Their size and shape varies, as does the experience of having them. Some people pass kidney stones without even realizing they’re there. Others may need to go to the hospital.

And then there are gallstones, which can be as small as a grain of sand or as big as a golf ball. They’re made of hardened deposits of cholesterol and bile. Doctors don’t fully understand why they form, but one thing is clear: nobody wants them.

Bonus: Gastrolith

The word “gastrolith” applies to two very different types of rocks. Neither are found in the human body, but both are awesome.

The first type of gastrolith can be any kind of rock, as long as an animal eats it on purpose. Crocodiles, seals, whales, birds, and their dinosaur ancestors have all been known to intentionally swallow rocks. Scientists used to believe that the crocs and sea creatures swallowed stones as ballast, to keep them from popping to the water’s surface like so many enormous corks. In recent years, some researchers have pointed out that for this scheme to work, the animals would have to swallow literal tons of rocks, which they clearly are not doing.

Bird gastroliths are a much simpler story. After swallowing, they keep their gastroliths in a pouch called a gizzard. With no teeth, the birds can’t chew up their food themselves, so the rocks do the work for them, grinding up each bite before it’s sent down to the stomach.

The second kind of gastrolith is totally different, but equally amazing. These gastroliths are little nuggets or disks of calcium carbonate that form in the heads of freshwater crayfish. The animals are frequent molters, and shed their shells many times over a lifetime. For a few days before they molt, the crayfish’s shell loses calcium, which goes into forming these gastroliths. After peeling off their tough-but-too-small exteriors, but before their new shells harden,  the arthropods are tired, floppy, and dangerously vulnerable. To speed up the hardening process, they reabsorb the calcium that they stored in the gastrolith, like personalized Flintstones vitamins. It probably goes without saying, but humans decided that this second kind of gastrolith was magical, or at least medicine.