Many people believe that diamonds are formed from the metamorphism of coal. That idea continues to be the "how diamonds form" story in many science classrooms.
It's easy to see where the idea came from, though. Diamonds and coal are both, at their base, different forms of the element carbon (C on the periodic table). And yes, pressure is a key part of what turns decaying carbon-based life forms such as plants into coal, as well as what turns carbon into diamonds. But the reality is just a little bit more complex than Superman's super-strength.
First of all, let's look at the chemical compositions of these two forms of carbon. Diamonds are essentially pure carbon formed into a crystalline structure. The rarer, colored diamonds do contain minor impurities (boron, for example, makes diamonds blue, while nitrogen turns them yellow), but those impurities exist on a scale of just one atom in a million.