Israeli scientists tell you why

While everyone has accepted the diamond as the hardest substance on earth without knowing the reason why, a team of Israeli scientists has given it a logical explanation.
The team from Israel's Bar Ilan University analysed the diamond’s construction. Professor Shmaryahu Hoz of Bar Ilan's Chemistry Department said that "The diamond is composed of pure carbon. The reason for it’s hardness is the connection between its carbon atoms. In the diamond every carbon atom is connected to four additional atoms, each of which is connected to three other atoms in addition to the initial one. Thus, each atom is surrounded by 12 atoms which are not connected to it and which reject each other”. Professor Hoz notes that this unique structure, which is accompanied by the mutual rejection between the atoms, is the explanation for the diamond's hardness.