I recall that the difference between graphite and diamond is in the arrangement of the carbon atoms and the strength of the carbon-carbon bonds. Diamond has a covalent bond; graphite has van der Waal bonds. I recognize "adamantine" as a description of "luster," so I'm not sure an "adamantine configuration" is. My Manual of Minerology does not go into enough detail.
Because of the carbon tetrahedrons in diamond, there is a lot of "open space" between the atoms. I think this accounts for diamond's cleavage. (Graphite, one the other hand, tends to be sheet-like.) So in diamond, the carbon bonds are strong, but as the crystaline structure is removed from the great pressures and temps of the mantle, the structure becomes unstable.
Sorry I don't have my crystalogrpahy book handy. It is out in the garage in a box somewhere.