Fig. shows an essentially identical experiment on a diamond NP, but with a very different result. The NP had an initial mass of ~17.3 MDa, which, as discussed in the SI, almost certainly indicates that the NP was an aggregate of smaller (~1 MDa) nanodiamonds. The 1900 K sublimation rate was reasonably constant at -528 Da/sec, corresponding to Rsublimation ≈ 0.27 Da/sec/nm 2 . The Rsublimation values for the six carbon NPs of five different feedstocks (Figs. 1A, S2-S4, and S6) varied by only a factor of ~two, and all fall within the range observed previously for 1900 K sublimation of 20 different graphitic NPs. In contrast, the 1500 K oxidative etching rate for the diamond NP was far slower than the etching rates for any of the other carbon NPs, as shown by the tiny change in the slope of M vs. time when the diamond NP was exposed to O2 (cyan background). The (Rbasecorrected) EEO2 value was only ~4 x 10 -6 Da/O2 collision, i.e., orders of magnitude smaller than the EEO2 values for the graphite, graphene oxide, carbon black, and nano-onion NPs under identical conditions (Figs. and). These data, therefore, raise two questions: 1. Why the diamond NP was so unreactive compared to all the other carbon NPs. 2. Why the 1900 K Rsublimation values were so similar for all the NPs, including the diamond NP which was nearly inert to O2.