The present invention pertains to a method for producing a colloidal suspension of zirconia particles, including successively subjecting a mixture of zirconium oxychloride and an alkali metal halide in an aqueous solvent to hydrothermal treatment at a temperature above 150° C, separating the slurry obtained from the supernatant, and peptizing said slurry by adding a strong acid thereto, and desalting said slurry, for example by ultrafiltration, so as to form a colloidal suspension of zirconia. The present invention also pertains to the transparent colloidal suspension obtainable by this method, and to an optical article, such as an ophthalmic lens, comprising a transparent polymer substrate and at least one coating prepared from a composition comprising said colloidal suspension.