|Fig. 1: Topaz gemstones of different colors. (Source: Wikimedia Commons)|
Topaz is a an aluminium fluorosilicate, whose chemical formula can be written as Al2SiO4(F,OH)2. The most common form of topaz is the colorless form, but it can take various different colors, for example, blue, pink, yellow and reddish brown, as can be seen in Fig. 4.  One of the most desirable is topaz with a medium blue color, called "London Blue".  The use of irradiation to produce such blue topaz from colorless topaz was first described by F. H. Pough in 1957, and it was rediscovered by Nassau in 1974. As a result, hundreds of thousands of carats of blue topaz entered the world market from 1975 to 1985. 
Whereas pink topaz is understood to be caused by the presence of chromium, the causes of the other colorations of topaz are not well known, although iron has been suggested as the cause of the blue color.  In gem minerals such as topaz, two major color mechanisms are known. The first is related to the presence of transition metals: this can occur when the metal ions transfer charges between themselves or to a neighboring 'O' atom. The second can be explained by color centers. These color centers are either vacancies within the structure of the crystal, such as in the case of quartz, where the Al3+ impurity creates a hole localized among its oxygen neighbors, or changes in the oxidation states in the cations and anions in the crystal. 
One can therefore surmise that it is possible to change the color of gemstones such as topaz, either through physical methods or through chemical methods. Indeed, if one were able to fill up the vacancies causing the colorations or change the oxidation state of the constituent cations and anions, one would be able to change the nature of the color centers and thus the color of the crystal. Indeed, it is well known that heating colored topaz can cause the topaz to either change its color or return to its colorless state. For example, the yellow to brown component of topaz can be removed through heating to about 200°C to 300°Cs, while heating to 500°C for a few hours can turn natural blue topaz colorless: in general, colors are lost upon heating from 400°C to 750°C.  Another method, first reported by F. H. Pough in 1957, would be to irradiate the topaz, thus releasing electrons from their original location in the gem: their eventual destination and the resulting change in oxidation number of the atoms around them will decide the eventual color of the gemstone. 
There are three main ways in which we can change the color of topaz: using gamma rays, high energy electrons and neutrons from nuclear reactors.  A brief outline of each method is given below:
Gamma rays are typically obtained from the decomposition of cobalt-60 atoms. The rays are very penetrating and, if the material is uniform, will produce uniform coloration.  Since relatively little heat is generated during the irradiation, there will be no thermal decomposition of the topaz, nor any degradation of the color produced. At high radiation doses, a green color may form as a result of the creation of a yellow to brown range of colors. Heating at 200°C - 300°C removes this coloration while keeping the blue intact, thus giving a uniform blue color to the gemstone. 
High energy electrons may be produced in a variety of ways, of which the one used by the jewelry industry is the linear accelerator, or "linac" for short, which is an electron gun that fires a pulsed beam of electrons at energies of 10 to 15 MeV with a beam currently approaching 1 mA. Unlike gamma radiation, this method produces significant heating, and the gem must thus be water-cooled to prevent cracking and melting. The large amount of electricity involved may also cause internal discharge, similar to a lightning, damaging the crystal in the process. 
Finally, there is the use of high-energy neutrons formed in nuclear reactors. This has been well documented by Leal et al., who found out that the blue color induced by neutrons in the gemstones is independent of the origin of topaz and correlated with an O- defect, while the concentration and blue color are increased nearly linearly by dose.  The colors produced are usually uniform and deep, darker than that produced by electrons, but often described as "inky" or "steely".  The disadvantage is the presence of residual radiation: blue topaz typically takes two to three years to decay below the exemption level after neutron irradiation. 
One might imagine from the above that gemstone irradiation is a very delicate and expensive process, which might account for the high market prices of gemstones. This, however, is not the case. One reason is that the supply of mined colorless topaz is larger than the demand for blue topaz, and there are sufficient amounts of unmined colorless topaz suitable for treatment if the demand should arise. Another thing to note is that there is no dedicated facility for the irradiation of gemstones: gemstone irradiation is conducted between other industrial irradiation processes such as medical supply sterilization and food preservation.  One might, however, hope that future advancements in the area of gemstone irradiation can lower the prices of highly-priced gemstones such as "London Blue" topaz, thus rendering them more accessible to the masses.
© Anrong Lin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 K. Nassau , "Altering the Color of Topaz", Gems Gemol. 21, No. 1, 26 (1985).
 J. Zhang et al., "The Radioactive Decay Pattern of Blue Topaz Treated by Neutron Irradiation", Gems Gemol. 47, No. 4, 302 (2011).
 K. Nassau and B. E. Prescott, "Blue and Brown Topaz Produced by Gamma Irradiation," Am. Mineral. 60, 705 (1975).
 A. S. Leal et al., "Study of Neutron Irradiation-Induced Colors in Brazilian Topaz", Nucl. Instrum. Meth. A 580, 423 (2007).`