This investigation is concerned with the characterization of the western desert porcelanite rocks; whose major mineral constituent is Opal-CT, and Nlnivite rocks; whose major mineral constituent is quartz. These materials were never been used for dielectric purposes. Accordingly, this investigation tries to examine their dielectric properties as well as the other related properties such as particle size distribution, porosity, densification, shrinkage, structure and thermal conductivity. The present work divides the study of these two types of rocKs into 24 groups according to weignt percent of'mixture, particle size distribution, heat, and chemical treatments. The conclusions drawn from this investigation were: 1-For western-desert porcelanste. A-Groups M and Ml give an acceptable sample, which, successfully can be used for dielectric purposes. This conclusion is drawn after comparison with values defining electrical insulators, with respect to their dielectric properties. B-Due to their lower volume shrinkage, samples from groups M4, M12, M13 and M14 give better properties than those found at group M and M1. C-Measurements at 100 'JC showed that the dielectric constant and dielectric loss index changes were minors. 2-For Ninivite materials: A-Group M15 shows an acceptable dielectric and thermal conductivity values. However, the volume shrinKage value shows higher value in comparison with samples accepted from the western desert area. 3-using tne volume shrinkage as a monitoring parameters, the groups M21, M24, M19 and M23 give dielectric values which are in the range of well-defined electrical insulators. The volume value of these groups is comparable with tnat of groups M and M1 Finally, this worK showed that, for laboratory scale, samples prepared from powder of particles of size distribution (<75µm) are generally accepted as electrical insulators, i.e. There is no needs to go for narrower distribution (<50 µm). This conclusion still requires furtner investigation if it Is to be applied for industrial purposes.