Abstract Stable colloids of hard magnetic particles are newly developed and very promising materials. Surface functionalization of these particles remains challenging because the particles tend to aggregate during reaction due to strong magnetic interactions. Herein we report on a synthesis of strontium hexaferrite hard magnetic nanoparticles coated with silica by hydrolysis of tetraethoxysilane. As a source of hexaferrite we used stable colloid of plate-like nanoparticles with mean diameter of 40 nm and thickness of 5 nm, which were prepared by a glass-ceramic process. We have shown that to successfully coat each hexaferrite particle individually the hydrolysis conditions should provide heterogeneous nucleation of silica with rate higher than the aggregation rate of the colloidal nanoparticles. The resulting materials represent single crystal hexaferrite cores wrapped in silica shell with mean thickness of 18 and 23 nm depending on synthesis conditions. The obtained core-shell particles can be easily dispersed as stable aqueous colloids. The materials can be used as magnetic sorbents or nanocontainers and, furthermore, they are very promising colloidal building blocks for various magnetically assembled nanostructures.