The refractive, transmissive or diffractive optical elements are made from a ceramic containing one or more oxides of the type X.sub.2O.sub.3, which is transmissive for visible light and/or for infrared radiation and which has a cubic crystal structure analogous to that of Y.sub.2O.sub.3. In preferred embodiments X is Y, Sc, In, or a lanthanide element, namely La to Lu, and in particular is Lu, Yb, Gd, or La. Also mixtures of oxides of the type X.sub.2O.sub.3 with oxides having different stoichiometries, such as HfO.sub.2 and/or ZrO.sub.2, may be present, as long as the cubic structure of the ceramic is maintained.