The rate of conversion of diamond to graphite is so slow, however, that a diamond persists in its crystal form indefinitely.
As temperature rises, the rate of conversion to graphite increases substantially, and at high temperatures it becomes (thermodynamically) favourable if the pressure is sufficiently high.
(They received the 2010 Nobel Prize in Physics for their work.) The greater degree of compactness in the diamond structure as compared with graphite suggests that by the application of sufficient pressure on graphite it should be converted to diamond.
At room temperature and atmospheric pressure, diamond is actually less stable than graphite.
When an element exists in more than one crystalline form, those forms are called allotropes; the two most common allotropes of carbon are diamond and graphite.
The crystal structure of diamond is an infinite three-dimensional array of carbon atoms, each of which forms a structure in which each of the bonds makes equal angles with its neighbours.
Other related properties are softness and lubricity (smoothness, slipperiness).
At the same time, however, the rate of conversion decreases as the (thermodynamic) favourability increases.
Thus, pure graphite does not yield diamond when heated under high pressure, and it appears that direct deformation of the graphite structure to the diamond structure in the solid state is not feasible.
A less common form of graphite, which occurs in nature, is based upon an ABCABCA… The amorphous varieties of carbon are based upon microcrystalline forms of graphite.
The individual layers of carbon in graphite are called graphene, which was successfully isolated in single-layer form in 2004 by physicists Konstantin Novoselov and Andre Geim.