The effect of the addition of small amounts of Y2O3 or CeO2 to a parent glass of the composition 21.2 MgO center dot 21.2 Al2O3 center dot 51.9 SiO2 center dot 5.7 ZrO2 (mol%) on the course of crystallisation and the mechanical properties of the respective glass-ceramics is examined. Depending on the applied heat-treatment procedure, glass-ceramics with Young's moduli and bending strengths that are significantly better than those of the parent glass can be achieved. Using X-ray diffraction and scanning transmission electron microscopy including energy-dispersive X-ray spectrometry, the glass-ceramics are characterised in terms of temperature-induced microstructural changes and crystal phase evolvement. It is shown that the addition of 0.5 mol% Y2O3 leads to glass-ceramics with less advantageous mechanical properties than those of the additive-free material. Adding 0.5 mol% CeO2 to the parent glass, however, does not affect the strong improvement of the mechanical properties upon crystallisation. Glass-ceramics with Young's moduli of up to 122 GPa and bending strengths of up to 350 GPa can be achieved this way. The addition of CeO2 has the advantage of adjusting the glass-ceramics colour to a certain extent, making it a promising candidate for artificial tooth replacement materials.