The invention relates to nanoparticles based on a metal, on a metal alloy or on a metal derivative coated with a silica layer, in which the silica is crystalline silica, in particular crystalline silica in the cristobalite form. They may be prepared from nanoparticles based on amorphous silica by making the latter crystallize by a heat treatment in air or in an inert atmosphere at a temperature between 900 and 1400°C and they may be used as pigments for ceramics or in the biological field.