Abstract: Quartz glass element, such as a diffusion tube useful in the production of semiconductor elements, capable of forming an outer layer of uniformly fine crystalline silica such as cristobalite or tridymite when heated to a temperature at which such crystalline silica forms containing crystallization promoting nuclei having a rate of diffusion in quartz glass less than that of sodium at elevated temperatures. Such nuclei are preferably present in the outer half of the element wall. When the quartz glass element is exposed to elevated temperatures, the nuclei promotes the formation of the outer layer of uniformly fine crystalline silica which imparts thermal dimensional stability for extended periods of use at elevated temperatures.