This is my own understanding of the process of diamond formation, which I have not heard anywhere else. It is believed that enormous amount of heat and pressure is required to form diamonds, but no explanation has been given why. What made me curious was the fact that, unlike fossil fuel, which is produced due to same process requiring pressure and temperature but absorbs some of the energy in its chemical bounding and released when burnt, the energy and heat spent in producing diamonds is not stored in the end product. So heat and pressure is not an essential requirement for the formation do diamonds. Understanding that will show that diamond structure is no more difficult to produce that graphite if approached in the correct way. Here I explain why:

Carbon atom vibrates at room temperature like other atoms. Since carbon is not a noble gas, it will combine with another carbon creating a strong covalent bound, if there are no other elements present. Due to the shape and number of free electrons, by default, carbon atoms combine in a hexagonal manner. Even though there are at least five different structures carbon atoms can combine together, at room temperature the graphite structure always has precedence over any other ones. If we subject the gas to some heat that agitates the carbon.

Now, if the carbon atoms are subject to increasing levels of heat, it will agitate the atoms until a point where the bounding of atoms can no longer hold and the atoms start moving around freely either as gas or liquid (depending of the element). So as long as the temperature is kept above that threshold, the carbon atoms do not form into graphite. But since there are other atomic structures that which are far more stable, but increasing the heat the chance of the atoms forming at least one of that type becomes likely. That is, since the temperature is high enough to stop graphite forming the only possible structure possible would be a diamond lattice. Since the atoms are free due to high heat that becomes a possibility. The reason why pressure has to be also high, is that the atoms move away from each other when the temperature gets higher, so by exerting pressure it keeps the atoms together. So over time if only 5 atoms form a diamond structure that initiates the processes. The heat and pressure no longer can dislodge those 5 atoms. So if the heat and pressure is kept high enough gradually the rest of the atoms add to that seed and become stable and stop agitating until all the atoms in that bubble is turned into diamonds, after that the heat and pressure has no effect on the diamond.

A diamond has a free unoccupied bound at every carbon atom on its surface, so it completes it with a layer of Hydrogen. So when you touch a diamond you are in fact feeling a layer of hydrogen rather than carbon as in graphite.

So the real this is the real reason why heat and pressure are required. Because the diamond structure is stable at a temperature that is too hot for graphite structure is hold its form. Since when diamond structure is formed it becomes extremely stable it will remain a diamond for ever.

I like to give an analogy to visualise this in another way. If a room with no air flow you get dust settling down. If there is slight air flow, it agitates the dust so it is unable to settle, but if you look in a corner of the room you see fluff of dust building up. That is because that fluff has found a stable structure that cannot be disturbed by that level of air flow. If the air flow increases, that fluff also gets broken up and something much more stable may take form. That is how trees evolve. Over millions of years the ideal design survives and the weaker one gets destroyed by the wind. So in places here wind level never gets high those designs do not exist.