[Lucy Lake]

Given enough time, all diamonds will become graphite at STP, fit only as a pencil lead.

How depressing. Did you have to post this!!!?

[RightWhale]

Diamond will replace coal at the municipal power plant. Clean-burning, clean as natural gas, even cleaner.

[capitan_refugio]

Well, there are always rubies and sapphires (corundum - Al2O3; hardness = 9)! Not to mention emeralds (beryl - Be3Al2(Si6O18); hardness = 7.5-8).

According to Hurlbut and Klein, Manual of Mineralogy, 19th Edition, 1977:

"The synthesis of diamond in 1955 was the realization of an age-old dream and established the stability relations between polymorphs of carbon over a wide range of pressure and temperature. Diamond, as to be expected from its high specific gravity and fairly close packing, is the high pressure polymorph. At low pressures or temperatures it is unstable with respect to graphite and may be converted to graphite at moderate temperatures. The reason that diamond and graphite can coexist at room temperatures and pressures is because the reconstructive polymorphic reaction between the two minerals is very sluggish."

So the good news is, your diamonds are probably okay for several million years!!