Everybody knows that diamonds are a girl's best friend, sang Marilyn Monroe, but until chemists at Bar-Ilan University and mechanical engineers at the Technion, Israel Institute of Technology studied them, nobody could explain the precious gem's secret for being the hardest material in nature. The answer to this question has been sought for years by numerous scientists around the world. In an article soon to be published in The Journal of Physical Chemistry by Prof. Shmaryahu Hoz of Bar-Ilan and of the Technion in Haifa with Dr. Lior Itzhaki, Prof. Eli Altus and Prof. Harold Basch of Bar-Ilan in Ramat Gan, the secret behind the super-hard material will be revealed. The researchers predicted that the discovery will lead to the development of additional super-hard materials, some of which may be even harder than the diamond. The authors noted that understanding the origin of materials' hardness is required for intelligent design. Their research resulted in the discovery that the interactions among atoms that are not chemically bonded directly to each other are responsible for the diamond's hardness. Hoz noted that diamonds are made from pure carbon. Every atom of carbon in a diamond is surrounded by four additional carbon atoms directly bonded to it. However, each of these atoms is linked to three additional carbon atoms. Thus the resulting structure was very special, he said. The team members, who for years have been studying the laws of mechanical engineering in the nano-metric world, previously discovered a synthetic molecular rod 40 times harder than diamonds and traced its hardness to its bonding hybridization. However, they said, this did not apply to the diamond, because the bonding hybridization in the gem was the weakest in the realm of carbon chemistry. Combining quantum mechanics and mechanical engineering at the nano-molecular level, they discovered another contributor to hardness: The repulsive interaction between two atoms not directly bonded chemically to each other. In their research, they found that this interaction is behind the mystery and is one of the major factors behind the diamond's super-hardness. The hardness results from the fact that diminishing the distance between these atoms induces an exponential rise in energy, the authors suggested.