Here's why:

* Diamond is made entirely of carbon atoms. It's a crystalline form of the element carbon, with the atoms arranged in a specific, very strong, and tightly packed structure.

* Compounds are formed when two or more different elements chemically combine. Since diamond is only made of one element (carbon), it's not a compound.

Think of it this way: You can't break down a diamond into simpler substances like you could with a compound like water (H₂O), which is made of hydrogen and oxygen.