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The "lowest energy state" (ie the form which the carbon atoms would prefer to arrange themselves, under room temperature and pressure) of carbon is graphite (layers of carbon atoms arranged in hexagonal sheets).

There is a significant "energy barrier", however, that separates the graphite phase of carbon from the diamond phase - to get to their preferred state, some energy must be externally provided to overcome this barrier.

Here is an analogy: you pay $400/month rent at the west end of town, but there is a place in the south end for $300/month that is just as nice and otherwise equivalent in every way to where you are now. So that *would* be your preferred "state". But there is a cost to moving there that you have to pay, so some additional effort has to be imparted to get there (ie there is a small "hill" in your costs - a one time expense, but after that, you are in a "lower energy state"). Never tried that one before, so I don't know how confusing it is (or if it was even needed!).

But anyway, once the energy barrier is overcome, it is much more difficult for the carbon to re-arrange into diamond (extreme temperatures and pressures, or very involved chemical processes). So, with enough time, one would expect all diamond to at some point be exposed to enough energy to overcome the barrier and turn into graphite. And once it is graphite it stays that way.

So: diamonds are not forever - they eventually will turn into graphite, which *is* forever.

:)