The invention proposes a method for producing glass ceramics which is particularly well suited as light conversion material, especially for down conversion. One initially produces a starting glass, containing (on an oxide basis) 5-50% by weight of SiO2, 5-50% by weight of Al2O3 and 10-80% by weight of at least one oxide selected from the from the group formed by Y2O3, Lu2O3, Sc2O3, Gd2O3, Yb2O3, Ce2O3, as well as 0.1-30% by weight of at least one oxide selected from the group formed by B2O3, Th2O3, and oxides of the lanthanoids, except Lu2O3, Gd2O3, Yb2O3, Ce2O3. Thereafter, the material is heated up for ceramization at a heating rate of at least 100 K/min to a temperature in the range of between 1000° C. to 1400° C. until crystallites are formed that contain a garnet phase. Thereafter, the material is cooled down to room temperature. Alternatively, controlled cooling-down from the molten state is possible.