Gemstone coloration

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. 

Because of the decay times needed, the trader s investment risk in the virgin material is a long time compared to the gem trade s cultural time frame. The reactor facility has an investment risk in the irradiation devices and the development of the measuring equipment. The facility is also dependent on the delivery of good quality virgin material so that it is not left with a large quantity of material that is useless because of long lived activation products. 

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. 

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But gemstone color is due to very small impurities in the mineral. For example, the mineral known as corundum is colorless when pure. But a very small amount of chromium produces a bright red color. The corundum is now a ruby. A touch of iron or titanium produces shades of yellow, green, purple, pink, or blue that turn it into a sapphire. 

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. 

Color and clarity are two of the attributes that give gemstones used ia jewelry value. Gemstones deficient ia either color or clarity can be enhanced (1). Almost worthless material can at times be converted iato valuable-appeariag gemstones. An estimated two-thirds of all colored gemstones used ia jewelry have been treated. Accordingly the identification of the use of treatments and the disclosure of enhancements to the purchaser are important. Table 1 Hsts the materials discussed hereia. 

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. 

Colored Impregnations. Colored oil to fill cracks is used on gemstones primarily to improve color, most frequendy on emerald. Colored oil is also used to simulate other stones, most frequendy quartz. Depending on the dye used, the colors may fade, in addition to the problems associated with colodess oiling. 

Composite Gemstones. Many types of composite or assembled gemstones have been made (1). Some are shown in Figure 3. In the United States a doublet has two pieces combined using a colodess cement. If three pieces are used, or if two pieces are assembled using a colored cement, the gemstone is a triplet. The use of composite stones has declined rapidly with the rise of inexpensive synthetics. Frequendy seen are opal doublets, where precious opal is backed by a black material. In opal triplets a thin sHce of precious opal is cemented between a black backing and a clear cover, usually of quartz. Additionally, insects and even fish have been inserted into amber. 

A homogeneous mixture of two or more components, whether solid, liquid, or gaseous, is called a solution. Solutions have variable composition while pure substances do not. That is, the relative amounts of the various components in a solution can vary. Thus, air, salt water, and sixteen carat gold are each solutions. The gemstone, ruby, is also a solution since it consists of the mineral corundum (AI2O3) with some of the aluminum replaced by chromium to give the crystal its characteristic color. Since the amount of chromium present can be varied, ruby is a solution. 

The two extremes of ordering in solids are perfect crystals with complete regularity and amorphous solids that have little symmetry. Most solid materials are crystalline but contain defects. Crystalline defects can profoundly alter the properties of a solid material, often in ways that have usefial applications. Doped semiconductors, described in Section 10-, are solids into which impurity defects are introduced deliberately in order to modify electrical conductivity. Gemstones are crystals containing impurities that give them their color. Sapphires and rubies are imperfect crystals of colorless AI2 O3, red. 

Gemstones can usually be identified and characterized by a number of physical properties if they are minerals, mainly by their crystal structure, but also by their density, hardness, color, and other physical properties. All gemstones of any one type of mineral have the same crystal structure (see Textbox 21), but the crystal structure varies from one type of gemstone to another. 

Mineral gemstones that have the same basic chemical composition, that is, are composed of the same major elements and differ only in color, are considered as variations of the same mineral species. As gemstones, however, minerals that have the same composition and crystalline structure but exhibit different colors are classified as different gemstones. Beryl, for example, a mineral (composed of beryllium aluminum silicate), includes a pink variety, known by the gemstone name of morganite, and also a well-known green variety, emerald. Table 18 lists and classifies, by composition and color, gemstones that have been appreciated since antiquity. 

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