What makes them different is their crystal structure:

* Graphite: has a layered structure where carbon atoms are arranged in hexagonal rings, forming sheets that are held together by weak van der Waals forces. This structure makes graphite soft, slippery, and an excellent conductor of electricity.

* Diamond: has a three-dimensional network structure where each carbon atom is bonded to four other carbon atoms in a tetrahedral arrangement. This rigid and strong structure makes diamond the hardest known natural substance and a poor conductor of electricity.