The melting point and boiling point, intensive physical properties. The melting point denotes the temperature at which a substance changes from a solid to a liquid. At the boiling point, a material changes from a liquid to a gas. These two characteristic temperatures of a substance are measures of the strength and the nature of the bonds that are formed within a substance.

Form	Melting Point (Celcius)	Boiling Point (Celcius)
Diamond	3550	4827
Graphite	3652 - 3697 (sublimes)	4200
Amorphous	3652 - 3697 (sublimes)	4200
Buckminsterfullerene

 

Are diamonds really forever?

Contrary to DeBeer's claims, a diamond is certainly not last forever. It is thermodynamically unstable relative to graphite at ambient conditions. Theoretically, the diamond you have is very slowly transforming into it's duller cousin - graphite. The most stable allotropic form of carbon is graphite. This is shown by looking at the free energy of formation, D G°f. The amount of energy required to transform one form of carbon to another can be determined by the difference between their heat of formation, D H°f, values. The degree of disorder is measured by the entropy value, S° ,of the carbon form. 
 
 

Form	D H°f (KJ/mole)	D G°f (KJ/mole)	S° (J/mole*K)
C (gas)	718.4	672.9	158.0
C (diamond)	1.88	2.84	2.43
C (graphite)	0.00	0.00	5.69
C (C60)

 

How extreme must the conditions be to produce diamonds?

Diamonds require extreme pressures and temperatures to form. They are formed at a depth of between 100 km and 200 km below Earth's surface, with a temperature range of 900 to 1300 degrees Celsius and pressures of 45 - 60 kBars. At ambient conditions (25 degrees Celcius and 1 Bar*) diamond is metastable. We can hold diamonds in our hands because its transformation to the graphite, the more stable form of carbon at ambient conditions, is slow. The temperatures and pressures at which the various forms of carbon are stable is shown below: