Diamond and Graphite

Diamond and graphite are both crystalline forms of the element carbon. Based on this fact, one would think they would be similar in many ways. But in fact they are very different. This highlights some important facts about crystals.

Both diamond and graphite have a very simple chemical composition; they are both pure carbon. Yet diamond is the hardest mineral known to man (10 on the Mohs scale), and graphite is one of the softest (less than 1 on the Mohs scale). As a result, diamond is the ultimate abrasive, whereas graphite is an excellent lubricant. Diamond is an electrical insulator while graphite is a good conductor of electricity. Diamond is usually transparent, but graphite is opaque.

Diamond and Graphite Structure

Diamond is obviously far more valuable than graphite. Graphite is so inexpensive that it is used to make pencil lead. Nonetheless graphite is as interesting a material as diamond in many ways. Graphite is the high-strength component of composites that are used to build automobiles, aircraft, high-tech golf club shafts and tennis racquets.

But why is diamond so hard and graphite so soft? Also, if graphite is so soft, how can it be used as a high-tech material for golf club shafts and aircraft?

The answer lies in the different atomic structures of diamond and graphite. Graphite forms in layers or sheets where the carbon atoms have strong bonds on the same plane or layer, but only weak bonds to the layer above or below. The carbon atoms in diamond, on the other hand, have strong bonds in three dimensions. In diamond, the atoms are very closely packed and each atom is connected to four other carbon atoms, giving it a very strong and rigid structure in three dimensions.

In the case of graphite, only the bonds between the graphite layers are weak. This means that when the layers or sheets are rolled up into fibers, and those fibers are twisted into threads, the true strength of the bonds becomes apparent. The threads are molded into shape and held in place by a binder such as an epoxy resin. The resulting composites have some of the highest strength-to-weight ratios of any materials.

The differences between diamond and graphite highlight the importance of crystal structure to the properties of a gemstone. For example, all gems that have a cubic crystal habit, where the crystal structure is completely symmetrical, have an important property in common. Cubic solids refract light in all directions at the same velocity. So even though diamond, garnet and spinel have quite different chemical compositions, they are all singly refractive because of their similar crystal structure. Other less symmetrical crystal groups are doubly refractive, including orthorhombic, monoclinic and triclinic, all of which have specific internal symmetries and axes that dictate how light bends as it enters the crystal.