I need some French help?

In my French books it says, "the material goes after the clothes word. You have to also add "en" before the material. Example: une chemise en coton." So for my practice sentence I did, "Pierre porte un manteau en cuir. But I was wrong it's supposed to be Pierre porte un manteau DE cuir. My question is why de instead of en? When am I supposed to use those words?