Gemstones optical properties

Diamond is supreme among natural gemstones ia H, RI, and DISP. Table 3 shows the steady improvement ia the sequence of diamond imitations, the aim being to produce a colorless, adequately hard material having closely matching optical properties. The iatroduction of synthetic cubic 2irconia ia 1976 brought about a sufficiently close match. 

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. 

Optical Properties. The high refractive index (2.42 at 589.3 nm) and dispersion (0.044) are the basis for the brilliance and fire of a properly cut gemstone. The optical transmission out to 10.6 p.m for Type Ila diamonds makes possible windows for CO2 lasers and for devices such as were in the... 

Other optical properties of gemstones, which also determine their beauty and other characteristics that make some of them unique, include the way they disperse light incident on them (see Textbox 22), their refractive index, which is unique to, and characteristic of every type of gemstone and is often used for their identification (see Textbox 22), and their luster, adularescence, asterism, and brilliance. 

There are three types of gemstone materials as defined by the U.S. Federal Trade Commission (1) (/) natural gemstones are found in nature and at most are enhanced (see Gemstones, gemstone treatment) (2) imitation or simulated, fake, faux, etc, material resembles the natural material in appearance only and is frequendy only colored glass or even plastic and (3) synthetic material is the exact duplicate of the natural material, having the same chemical composition, optical properties, etc, as the natural, but made in the laboratory (2,3). Moreover, the word gem cannot be used for synthetic gemstone material. The synthetic equivalent of a natural material may, however, be used as an imitation of another, eg, synthetic cubic zirconia is widely used as a diamond imitation. 

Since many gemstones appear quite similar, which is why they can be simulated, it is important to be able to distinguish the real from the synthetic or from the simulant. If you have a scanning electron microscope (SEM) with XEDS available, the latter task is not a problem, but this is usually not the case in the field (or the shop). The basic tests are thermal conductivity, optical properties, and mechanical properties. Using a mechanical test to characterize a material that you do not want to damage is clearly tricky, so the use of hardness measurements is directed more toward minerals than gemstones. 

Andalusite, kyanite, and sillimanite are three polymorphs minerals that belong to the ne-sosilicate minerals. Hence they have the same chemical formula [Al SiOj = Al Oj-SiOJ and all contain theoretically 62.92 wt.% Al O, and 37.08 wt.% SiO. They are distinguished from one another by their occurrence and physical and optical properties (see Section 12.7, Minerals and Gemstones Properties Table). Kyanite is easily distinguished from sillimanite or andalusite by its tabular, long-bladed, or acicular habit and by its bluish color and slightly lower hardness than sillimanite and andalusite. 

Lithium niobate [niobium + -ate[ (1966) n. LiNb03. A crystaUine material whose physical properties change in response to pressure or the presence of an electric field and which is used in fiber optics and as a synthetic gemstone. 

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