Chemical element with atomic number 6 and with unique binding and crystallisation properties. Those are due to the 4 valence electrons of the atom.

4 Valence electrons allow for a big magnitude of possible binding and crystallisation forms. Each atom can altenatively accept or provide up to 4 electrons for bindings to other atoms.

By beeing able to form alternatively 4 single - electron - bindings or two 2-electron bindings or one 2-electron-binding plus 2 single-electron-bindings it can form molecules of different structure with any other chemical element except inert gases.

Carbon atoms are the basis for the whole world of organic polymers.

Carbon - carbon bindings without any foreign atom form the largest family of allotropes existing. Allotropes are molecules of identical chemical composition but different structure and properties.

The properties of carbon allotropes are tremendously different:

In form of furnace it is a completely black soft substance while in form of diamond it can be fully transparent and the hardest material existing.  But even a big variety of furnaces is existing, some of them electrically insulating, others conductive. Graphite is a further allotrope as well as amorphous carbon, graphen, fullerens and nanotubes, listing only a few of possible crystal forms. Some of them can integrate foreign atoms into their crystal grid, others don't.

Carbon crystals
8 Allotropes of carbon crystals