YELLOW SAPPHIRE: NATURAL, HEAT-TREATED, BERYLLIUM-DIFFUSED, AND SYNTHETIC

Natural yellow sapphires are colored by one of two entirely different chromophores or by a combination of the two. These two chromophores are iron (Fe3+) and a trapped hole paired with iron (h(center dot)-Fe3+). The color saturation of the Fe3+ chromophore, as previously documented, does not linearly depend on its areal density, unlike the other five chromophores in natural corundum. It exhibits a complex dependence on areal density, a relationship that is explored here. Low-iron-content natural yellow sapphires are colored solely by the h(center dot)-Fe3+ chromophore. The yellow sapphires from Sri Lanka are colored in this way. Some of the basalt-hosted high-iron sapphires from Australia and Thailand are colored by h(center dot)-Fe3+ in addition to Fe3+. Natural sapphires that are colorless or weakly yellow often develop strong yellow coloration via heat treatment. They depend on formation of the h(center dot)-Fe3+ chromophore for that change. In these natural stones that respond to heat treatment, one of two different internal chemistries is present, which must be altered to bring about the color enhancement. These two chemistries, which require two different heat treatment processes, are presented. The diffusion of beryllium into various types of sapphire can shift their chemistry from donor- to acceptor-dominated, forming the trapped hole, h(center dot), which pairs with iron to produce the intense yellow coloration. Although crystals with the h(center dot)-Fe3+ chromophore were grown as a part of our study, synthetic yellow sapphires are not often colored by the same chromophores of natural sapphires. They are usually colored by the Ni3+ chromophore or by Ni3+ and Cr3+. Somewhat surprisingly, the color saturation of Czochralski-grown nickel-doped sapphire can sometimes be enhanced by heat treatment.