In theory, diamond should 'degrade' to graphite at surface temperatures and pressures, as the latter form is the lower energy state of pure carbon, and matter tends to revert to its lowest energy state.
However, to re-arrange the carbon atoms in the lattice structure requires an initial boost of energy, known as the 'activation energy'. This is very high for diamond, and thus, the energy required to get the process going is never attained at (Earth's) surface temperatures.
For this reason, diamond (being the high temperature / high pressure allotrope of carbon, since it is formed at great depths within the Earth) is referred to as being meta-stable - meaning it should become graphite, but doesn't !!
So that means;
"A diamond is a girl's meta-stable best friend"