Diamond and pencil graphite are both made of pure carbon.

Diamond is one of the hardest natural materials known, while graphite — the “lead” in pencils — is soft enough to flake off onto paper with a light touch. Yet both are made of nothing but carbon atoms. The dramatic difference in their properties comes entirely from how those atoms are arranged.

In diamond, each carbon atom bonds tightly to four neighbors in a rigid three-dimensional lattice, which is what gives it extraordinary hardness. In graphite, carbon atoms form flat sheets, each atom bonded to only three neighbors, and the sheets stack loosely enough to slide past one another. Materials like these, sharing an identical chemical formula but strikingly different structures and properties, are called allotropes.