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Diamond, despite its high hardness, is the metastable form of the element carbon. By exceeding temperatures of above 2000 K [9], diamond transfers into the stable modification of graphite (Figure 2.7). The retransformation of graphite is possible under high temperature and high pressure [10].

The sp³ hybridization with four bondings arranges the adjacent C atoms in a tetrahedral lattice with a bonding angle of 109°, 47° (Figure 2.4). This is a cubic isotropic structure with high rigidity.

Hexagonal diamond was found as a by‐product of diamond synthesis and later in meteorites [11]. In the meantime, chemical vapor deposition conditions of methane/hydrogen gas mixtures at low pressure allow the production of synthetic diamonds.