Allotropy

Ability of a single substance to exist in more than one physical form.

Examples

Iron

• $\alpha-\ce{Fe}$ (Ferrite) - Below $912\degree \text{C}$
• $\gamma-\ce{Fe}$ (Austenite) - Between $912\degree \text{C}$ and $1394\degree \text{C}$
• $\delta-\ce{Fe}$ - Above $1394\degree \text{C}$
• $\epsilon-\ce{Fe}$ (Hexaferrum)

First 3 exist in atmospheric pressure. Forth one exists in very high pressure.

Carbon

• Graphite
• Diamond
• Carbon nanotubes

Buckminsterfullerene

Aka. bucky ball. Has $60\;\ce{C}$ atoms inside a molecule. A molecule is truncated icosahedron shaped. Diameter of the molecule is approx. $1\,\text{nm}$. Each carbon atom is bonded to another 3. Not chemically inert.