Popular culture has often been the source of a number of myths. Classic Superman comics help create the misconception that coal can be turned into diamonds. While no expects coal to turn into diamonds by squeezing a handful of it like Superman can, but there is the notion coal can turn into diamonds over the course of time. This is not the case at all. Coal cannot and never will turn into diamonds.
The origin of the myth might not be rooted purely in Superman comics, but a misunderstanding of basic science. Both diamonds and coal are a form of the element Carbon C. Carbon does turn into a diamond. Specifically, over many years, the carbon element turns into a crystalline form. Generally, the carbon element that becomes a diamond is pure. There are a few rare colored diamonds that might contain a minor amount of other elements, but they are mostly pure.
Coal, on the other hand, is about as far from pure as you could get. In addition to the carbon element in coal, there are scores of properties that find their way into the mix. These ingredients can range from oxygen to arsenic to sulfur to mercury and more. What might find its way into the coal is going to be based on where the coal is being mined. Furthermore, coal comes from the decay or organic materials. Bacteria, plants, and fungus all can create coal. How much organic material is in the coal is going to be based on the location where the coal is found. The reason coal, even though it has a carbon element to it, cannot become a diamond is there are just way too many impurities found in it to make this transformation possible. These impurities are also at the root of the environmental concerns about coal. When coal is burned, these impurities find their way into the air. Once there, all manner of problems can emerge.
Crushing pressure alone is not what makes a diamond. The weight of all the earth on top of the carbon is not all that creates a diamond. Heat has an enormous impact on the results. Basically, a combination of thousands upon thousands of degrees of heat combined with 130,000 atmospheres of pressure help create a diamond. Such a process usually occurs roughly 100 miles below the surface of the Earth. A molecule bond occurs between carbon and other atoms and diamonds develop.
Through erosion, eruptions, seismic shifts, and the like over millions of years, diamonds found their way closer to the surface. The closer they are to the surface, the easier they are to mine.