Abstract: Methods of isolating quartz, quartzite, glass or silicate materials having selected aluminum contents are described. The non-destructive method comprises subjecting the materials to ionizing irradiation for a period of time and at an intensity (preferably uniform) sufficient to develop the several distinctive color centers of the materials containing aluminum ions. According to the ions associated with the aluminum ions, the depth of color of each of the various tints developed corresponds to the amount of aluminum in said crystals. The colored crystals which correspond in tint and depth of color to the aluminum contents outside the desired content range can be separated. In this manner, aluminum free quartz, quartzite, glass or silicate materials can be isolated if present in a mixture, or fractions of such materials can be selected, each containing a uniform and known aluminum content.