Carbon has only two true crystalline forms: diamond and graphite.

The same chemical element can sometimes arrange its atoms in different structures. These different structural forms are called allotropes.

Carbon provided the classic example. In diamond, each carbon atom is locked into a rigid three-dimensional network. In graphite, the atoms form flat sheets that can slide over one another. For generations, those were the two principal crystalline forms of carbon taught in textbooks.

Then the family expanded.

In 1985, experiments that vaporized carbon produced C60, a molecule made from sixty carbon atoms joined into a hollow cage shaped roughly like a soccer ball. It became part of a new class called fullerenes.

Researchers later identified carbon nanotubes and in 2004 isolated graphene, a single layer of carbon atoms arranged in a flat honeycomb pattern.

Not every proposed carbon structure is equally stable or well established. But diamond and graphite are clearly not the only distinct ways carbon atoms can organize themselves.