Irradiation, gemstones

Several gemstone species occur in various colors, depending on the presence of impurities or irradiation-induced color centers. Examples are the beryl, comndum, and quart2 families. Quart2 has poor optical properties (RI = 1.55, DISP = 0.013), but becomes of gemological interest when it exhibits attractive colors. Any material can have its color modified by the addition of various impurities synthetic mby, sapphires, and spinel are produced commercially in over 100 colors (2). Synthetic cubic 2irconia has been made in essentially all colors of the spectmm (11), but only the colorless diamond imitation is produced commercially in any quantity. 

Table 3. Rays and Particles Commonly Used for the Irradiation of Gemstones...

With the exception of diamond coloring and the turning of topa2 blue, the source of the irradiation is immaterial. Gamma rays are the preferred source because of uniformity of coloration and the absence of heating and induced radioactivity. The most commonly seen gemstones enhanced by irradiation are summari2ed in Table 4. 

Radioactive Gemstones. Zircon can contain radioactive elements, but the amount in jewelry-grade material is insignificant. Some of the treatments of Table 3 may leave irradiated material radioactive. Such gemstones have been released on rare occasions without the required cooling-off period (10). 

Several gemstone species occur in various colors, depending on the presence of impurities or irradiation-induced color centers. Any material can have its color modified by the addition of various impurities synthelic ruby, sapphires, and spinel are produced commercially in over 100 colors. 

When radiation interacts with mailer, a displacement of the outermost electrons in atoms occurs. This displacement can lead to the formation of color centers or to valence slate changes, The most commonly seen gemstones enhanced by irradiation are summarized in Table 4. When properly performed, there is no significant residual radioactivity. 

TABLE 3 RAYS AND PARTICLES COMMONLY USED FOR THE IRRADIATION OF GEMSTONES... 

Nassau, Kurt (1984) Gemstone Enhancement Heat, Irradiation, Impregnation, Dyeing and Other Treatments Which Alter the Appearance of Gemstones, and the Detection of Such Treatments, Butterworth Heine-mann, London. Appendix A is a brief and clearly written description of the different types of furnaces often encountered in ceramics laboratories. 

Blue. The common blue stones are sapphire, lapis, and irradiated topaz. Less common gemstones include blue diamond, although such stones could be common if irradiated more often. 

Two methods that are widely used to enhance the color of gemstones are irradiation and heating, usually in that sequence. Table 36.11 summarizes some of the heat and irradiation treatments that have been used. The reason for treating (processing) gemstones is invariably to enhance their appearance and thus increase their value. We will review the general features of the different treatments and the science behind them, but refer discussion of the details to the sections on particular materials. All irradiated samples will be heated. 

Gemstones may be irradiated with neutrons to improve their properties (e.g., change to a more desirable colour) in order to increase their demand and monetary value. The most common neutron irradiation being performed at research reactors is for topaz. Therefore, the following information is pertinent to topaz irradiation. 

The world of gemstones and the world of reactor management, operation and utilization are culturally very different. The market for the stones is whimsical and difficult to assess. It is dependent on the size and the colour of stones currently in vogue. This often makes the behaviour of the gem trader also appear to be whimsical. In addition, the operating schedule for the research reactor and the time required for the irradiation and processing of stones is often incompatible with the needs of the trader. 

Nevertheless, several research reactors worldwide are irradiating topaz commercially and performing research on irradiation of gemstones, but only three facilities have applied for the license required in the USA. 

The colour in topaz is induced by the interaction of fast neutrons. If thermal neutrons are allowed to irradiate the topaz, then significant undesirable radioactivity will be induced in the gemstone. 

If the temperature of the gemstones approaches 300°C the damage to the topaz will anneal and de-coloration will occur. In addition, if the temperature is too high the stones will be prone to flaking during post-irradiation handling. A typical temperature during irradiation should be... 

---