We submit results of the study of hydrosols of the single crystalline diamond nanoparticles ranging of 4-5 nm, obtained by detonation synthesis. The stable hydrosols with negative and positive zeta potentials were obtained from industrial powder after additional purification and subsequent annealing in air or in hydrogen. Unusual behavior of the hydrosols was revealed with increasing concentration of diamond nanoparticles. The state is characterized by giant viscosity value and is reversible unlike SiO2 hydrogel. Formation of the state has been explained in the frame of model that assumes the polyhedral shape of diamond nanoparticles and consequently non-spherical surface charge distribution. Stability of hydrosol was determined in accordance to the Derjaguin - Landau - Vervey - Overbeek (DLVO) theory, although as it is clear from the DLVO theory, that the double electrical layer in hydrosol is not spherical in that case. This non-sphericity results in gel formation at a smaller threshold concentration than that of DLVO theory prediction. The suggested model is supported by experimental data on particle size distribution, obtained by dynamic light scattering, by small angle neutron scattering and by theoretical estimation of the threshold concentrations.