It is a spontaneous reaction but it's an incredibly slow one and the energy change is something like -1.1 J/mol which means it really doesn't care whether it's graphite or diamond, but if it had to choose it would choose graphite.
Shitty analogies by Tyler.
Perhaps not a self-sustaining fire, but you can absolutely char the surface of a diamond. In fact the oxidation of diamond is spontaneous at standard temperature (25 C).
C (diamond) + O2 --> CO2 (g) Gibbs free energy at 25C = -397.24 kJ/mol
Shitty analogies by Tyler.
C (diamond) + O2 --> CO2 (g) Gibbs free energy at 25C = -397.24 kJ/mol