The invention provides a method for producing a glass ceramic comprising the steps of melting a starting glass that is free from alkali, except for incidental contamination, and that contains at least one garnet-forming agent and at least one oxide of a lanthanoid; grinding the starting glass to produce a glass frit; molding by pressing and sintering the glass frit until at least one garnet phase containing lanthanoids is formed. A glass ceramic produced in this way may contain 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 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. Such a glass ceramic is suited especially for down conversion of excitation radiation in the blue and in the UV region of the spectrum.