Glasses in the system of MgO/Al2O3/SiO2 with different concentrations of zirconia as nucleating agent, some of them additionally doped with ZnO or P2O5, were annealed in a temperature range from 950 to 1150°C. The use of zirconia led to colorless glass–ceramics, which were transparent to opaque. In all studied compositions, α-/β-quartz-solid-solutions, zirconia as well as spinel or gahnite (ZnAl2O4)/spinel-solid-solution precipitated. The highest bending strength of 475 MPa was obtained after annealing at 1000°C for 3 h. The increase of the annealing temperature or an increase in the zirconia concentration resulted in an increase of the microhardness up to 13.3 GPa and of the fracture toughness up to 2.7 MPa m1/2. The addition of ZnO results in an increase of the hardness up to 12.5 GPa. The addition of ZnO or P2O5 led to a fracture toughness of 2 MPa m1/2. The described physical properties had to be highly advantageous for the preparation of colorless high strength dental glass–ceramics. © 2011 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 100B: 463–470, 2012.