A process for upgrading an inferior grade of zircon to a superior grade thereof includes mixing the inferior grade of zircon, in comminuted form, with at least one mineralizer, to obtain a zircon/mineralizer mixture, which is a calcined product. The calcined product is washed, and thereafter, in a comminution step, the washed calcined product is comminuted to obtain a superior grade of zircon, which is suitable for use as a glaze opacifier.