Why are Diamonds So Hard?


Any High School Chemistry student can tell you that diamonds and graphite are made from pure carbon. So why is one soft while the other hard? How can diamonds, the hardest naturally occurring substance known, be made of the same substance as soft graphite?


Of course the difference is how the atoms are linked together. Carbon is fairly common, being one of the most abundant elements in the Universe. Its atoms can link with many other elements in many different ways; there are about 10 million different known compounds that include carbon. These carbons atoms can also link together with each other.


Each carbon atom has four electrons in its outer shell. This outer shell can potentially hold 8 electrons. Carbon atoms can link together so that their outer shells share 8 electrons between them. Carbon atoms can also link with other atoms by sharing electrons with them, but as diamonds are made only of carbon we can concentrate on carbon linking with itself.


It is possible to link the carbon atoms together in many different patterns. We could have two atoms sharing 8 electrons, or we could have many atoms bonding together with shared electrons. With graphite the carbon atoms bond to form molecules in flat layers. The bonds within layers are strong, but the bonds between layers are very weak. As such graphite is soft because it consists of several very thin layers of carbon rubbing together.


By contrast, diamonds consist of a 3D lattice of atoms. Each carbon atoms is linked to several other carbon atoms in several directions. This lattice is equally strong in all directions; it is rigid and the atoms cannot move relative to each other. As such, diamonds are made of a solid lattice of carbons that is equally strong in all directions.


Why is the structure of the diamond atoms so different to the structure of atoms in graphite? The process involves a combination of several factors, but extreme heat and extreme pressure followed by rapid cooling is involved. Diamonds are relatively rare (at least near the surface of the Earth) because the right combination of several factors is required. But when the right factors do occur the resulting diamond will last for centuries.