Diamond Vs Graphite

C(diamond) + O2(g) → CO2(g) --DH = -395.4 kJ mol-1
CO2(g) → C(graphite) + O2(g) --DH = +393.5 kJ mol-1

Add the two equations together:
C(diamond) → C(graphite) --DH = -1.9 kJ mol-1
It is exothermic. Therefore, graphite is energetically more stable.

Carbon does not convert from diamond to graphite at room temperature. This is because the activation energy is very high, as the C-C bonds are very strong and a lot of energy is needed to break them.
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