The Heat and Diffusion Treatment of Natural and Synthetic Sapphires

The Seven Types of Yellow Sapphire and their Stability to Light

“…Irradiations were performed in a cobalt-60 gamma cell at a rate of one Mrad per hour (Nassau, 1983). In all instances, the color reached saturation, as visually determined, in much less than eight hours (the minimum time used).…”

“…Smith (e.g., in his 10th edition, 1949) said that yellow sapphire lost its color when heated and that radium (i.e., irradiation) either deepened the color or developed it in a colorless stone. More recent studies have expanded our knowledge of this behavior (Crowningshield and Nassau, 1981;Fryer et al, 1983aFryer et al, , 1983bKoivula, 1986;Nassau, 1980Nassau, , 198 1, 1984Schiffmann, 1981;Schmetzer and Bank, 1980;Schmetzer et al, 1983). Some studies (e.g., Schmetzer et al, 1983) have suggested that the yellow color may arise from color centers, or iron, or both.…”

“…Types 2 and 7, especially, can co-exist with other types; thus, a pale yellow type 1 sapphire may be irradiated to a dark orange color, which is then type 1 + 2, but in light this will fade back to the original type 1 pale yellow. All of these types of yellow can also coexist with a chro1niun1-produced pink, resulting in an analogous group of seven types of "padparadscha" sapphire (Crowningshield, 1983).…”

The Chanthaburi-Trat Gem Field, Thailand

“…These facilities treat not only the local production, but large quantities of material from Sri Lanka, Australia, Kampuchea, and Burma as well. Nassau (1981) and Crowningshield and Nassau (1981) examined the technical aspects of heat treatment and its detection in ruby and sapphire in quite some detail. They limited themselves to those questions relating to enhancing asterism or improving the color of ruby and blue sapphire, however.…”

“…Most of the sapphires treated are colorless stones from Sri Lanka. These sapphires are typically very clean and free from the "silk" or rutile that provides the titanium which, in conjunction with divalent iron, is responsible for a sapphire's blue color (Nassau, 1981). Any highly included or flawed portions of the rough sapphire are trimmed prior to treatment.…”