To become inert, an NP, or at least its surface layer, must transform to a structure with few under-coordinated sites. The fact that different carbon materials evolve at different rates presumably reflects the differences in the feedstock structure. For example, carbon black NPs become inert after losing only ~10% of their initial mass, and this may reflect the fact that primary carbon black NPs are roughly spherical, lamellar structures of graphene-like platelets, which could form nano-onions by linking the platelets together to form continuous layers. Thus, carbon black NPs would appear to require the least large-scale structural reorganization to "onionize".