Almost all materials will go through a phase transition when subjected to high pressures, among which carbon stands out for its flexibility to hybridize by sp, sp2 and sp3 bonds. In this work, we used glassy carbon (GC), a disordered amorphous form of carbon that is mostly sp2 bonded, as the precursor for compression up to pressures of over 100 GPa. The successful room-temperature synthesis of nanocrystalline diamonds, with both cubic and hexagonal structures, from GC was an exciting achievement. This discovery drives our study of the phase transition process from a disordered to a crystalline structure. By investigating the stresses that drive this transformation, we propose a shear-driven phase transition. Indeed, by compressing GC in a shear-free environment no diamond was found. The mechanical properties of samples after compression were investigated. We found several key pressure thresholds that help explain the evolution of structural changes under pressure. We observe that the hexagonal phase of diamond is not as hard as predicted but is very close to the hardness value of regular diamond. Molecular dynamics simulations also provide a good visualisation of these findings. Finally, finite element analysis is performed to model the indentation behaviour to help support the mechanical properties experiments.