Final answer:
The conversion of diamond to graphite is spontaneous under ambient pressure and occurs at an immeasurably slow rate at low temperatures. Graphitization can be accelerated at higher temperatures. Graphite is the stable form of carbon under ambient pressure.
Step-by-step explanation:
The conversion of diamond to graphite is indeed spontaneous under ambient pressure. However, the rate of conversion is very slow at low to moderate temperatures. This process, called graphitization, can be accelerated at temperatures in the 1000-2000 K range. The phase diagram for carbon indicates that graphite is the stable form of carbon under ambient atmospheric pressure, while diamond is stable at very high pressures.