So what are koi teeth made of?
Shark Teeth are actually scales, that's why they can infinitely grow them back.
Teeth are key to understanding prehistoric sharksIn fact, if it weren’t for shark’s teeth, we would know very little about prehistoric sharks at all! Since sharks are cartilaginous fish, their teeth are often the only part of their bodies that entered the fossil record. This is because shark teeth are modified placoid scales, which, like our own teeth, consist of a central pulp cavity surrounded by dentine and a hard enamel surface. Shark teeth are used almost exclusively to classify ancient…
https://oceansyrup.com/shark-scales-teeth/