Nanodiamonds obtained from detonation of explosives with negative oxygen balance were annealed under vacuum up to 1500 o C and turbostratic graphitic carbon (the largest X-ray peak corresponding to d 0 0 2 = 0.344 nm) was observed. High-resolution electron microscopic observation shows the complete transformation of crystalline nanodiamonds into up to 9 concentric shells (onion-like structure). Pentagonal carbon cycles have been identified in these onion-like structures by STM. This observation is consistent with former results showing the formation of quasi-spherical particles formed by closed carbon shells either in detonation or by irradiation of soot and tubular carbon by an electron beam. These results support the idea that the usual planar graphitic structure is not the only equilibrium structure of carbon under normal conditions; the shell structure could represent a solid carbon equilibrium structure.Des nanodiamants obtenus par detonation d'explosifs surcarbones dans un confinement d'eau ont ete soumis a un chauffage progressif sous vide jusqu'a 1500 o C et se sont intregralement transformes, au cours de ce traitement, en une structure de type graphitique turbostratique selon le spectre de rayons X (pic principal correspondant a d 0 0 2 = 0,344 nm). L'examen au microscope a haute resolution revele la transformation en coquilles fermees concentriques formees par six a neuf couches. Des cycles a cinq atomes de carbone sur les structures type oignon , resultant de l'evolution thermique de nos nanodiamants, ont ete observes. Ainsi, le retour sp 3 sp 2 peut former des structures en couche concentrique, type fullerene et non du graphite comme le prevoit le classique diagramme d'equilibre du carbone. Nos resultats, ainsi que les resultats d'autres auteurs, nous conduisent a penser que la structure d'equilibre du carbone en hybridation sp 2 n'est pas necessairement le graphite plan, mais une structure en coquilles concentriques (structure type oignon).