Glyph,
I know that diamonds can burn but I still doubt they will convert to graphite at room temperature and pressure because, as one of the articles you linked says, the activation energy for the reaction to occur is very high.
Somehow I doubt that graphite specks found in diomonds in the earth's crust formed at room temperature. I would think it's more likely that they were formed when the dimond was heated or as a result of other impurities when the diomond was formed.
Even with a high activation energy, if the product is more stable than the reactants, the reaction will proceed eventually. This is a scientific certainty. Although it may take eons, if something is unstable, it has to go.
The enthalpy of the reaction of diamond to graphite proceeds with a change of negative 2kJ/mol. Although small, this does prove that diamond is the unstable form over graphite.
As for activation energy. A high activation energy means the reaction is very slow, not that it can't happen at low energy. A first year chemistry text-book (i personally reccomend ones by Zumdahl) details this.
After digging through the scientific literature I found this article mentions the process as well as the activation energy:
Sacha Welz, Yury Gogotsi, Micheal J. McNallan, "Nucleation, growth, and graphitization of diamond nanocrystals during chlorination of carbides" Journal of Applied Physics, 4207, Vol.93
Using their calculations and numbers I came to a half-life, for pure diamond, of 3x10^15 years. Yes i realize this is a million times older than the universe but the point is, it does happen.
this website quotes the half-life at millions of years:
**broken link removed**
Although personally i think they pulled that number out of their ass.
Anyway keep in mind, this calculation is for uncatalyzed pure diamond. Nature however is not that clean, most diamonds contain impurities that catalyze the reaction. In fact this is how diamonds are made articially in a few weeks by including a catalyst material (catalysts speed up both directions of reactions, forward and backward).
So the decomposition of natural diamonds can be sped up by the impurities in it. Those specks in low-grade diamonds, while i agree some of them are probably incorporated crap, others are actually nucleation sites around impurities which are catalyzing the decomposition. The aforementioned article cites a dozen papers where scientists are actively studying nucleation and catalysts.
Bottom line is, Diamonds may outlast humans, perhapes humanity as a whole, and possibly our earth and our sun. But they are certainly not forever. I'll stake my Ph.D in chemistry on that.