In his book Principles of Chemistry Mendeleev wrote: The molecules of coal, graphite and diamond are very complicated, and carbon atoms exhibit the capability of binding one to another to form complex molecules in all compounds of carbon. None of the elements possesses an ability of complicating in such an extent as does carbon. There is still no basis to define the polymerization degree of the coal, graphite or diamond molecules. One should believe, however that they contain Cn species,“n” is a large value.Up to the middle of 20th century, only two allotropic forms of carbon were known, namely, graphite and diamond, including their polymorphous modifications. Indeed under ambient conditions and in bulk form, the graphite phase with strong in-plane trigonal bonding is the stable phase, as indicated by the phase diagram shown in Fig. 9.1. Under the application of high pressure and high temperature, transformation of the graphite structure to the diamond structure takes place. Once the pressure is released, diamond remains essentially stable under ambient conditions although, in principle, it will very slowly transform to the thermodynamically stable form of solid carbon, which is graphite. However, when exposed to various perturbations, such as irradiation and heat, diamond will quickly transform to the equilibrium graphite phase.