I know that coal (graphite) turns into diamond when a high enough pressure is applied, but I guessed it would stay as a diamond unless it was incinerated by a high enough temperature.
However, I recently took a closer look at the phase diagram of carbon, and it seems that diamond should turn back into graphite on low enough pressures even at room temperature:
I guess this is a relatively easy experiment to make, I just don't have the necessary funds to sacrifice a diamond, and I couldn't find such an experiment with a quick Google search.
What would really happen when a piece of diamond is put into a vacuum chamber and air is pumped out, as the pressure drops enough so that the subject leaves the "metastable diamond" phase? Does it remain the same? Does it just peacefully turn into a similarly shaped piece of graphite, which remains a normal piece of graphite after putting it back into atmospheric pressure? Or would something more spectacular happen?