A spatial transition of the carbon state in detonation nanodiamond (DND) from crystalline diamond inside the particle to a graphite-like state at the DND surface is proposed on the basis of small-angle neutron scattering (SANS) analysis. The SANS contrast variation from concentrated (5 wt%) dispersions of DND in liquids (water, dimethylsulfoxide) reveals a shift in the mean scattering length density of DND as compared to pure diamond, which is related to the presence of a non-diamond component in the DND structure. At the same time, the diffusive character of the particle surface is deduced based on the deviation from the Porod law. The two observations are combined to conclude that the continuous radial density profile over the whole particle volume conforms to a simple power law. The profile naturally suggests that non-diamond states are concentrated mainly close to the particle surface; still there is no sharp boundary between the radial distributions of the two states of carbon in DND.