The aim of the investigation was to develop an opal dental glass, to characterize its microstructure and to determine the thermal properties. It was possible to develop opal glasses derived from the system SiO2-Al2O3-K2O. The opalescent effect is comparable to that of natural teeth. During this study it was found that two mechanisms must be controlled : the phase separation in the glass and the minimum surface crystallization at glass grain boundaries within the monolithic sample.