Gemstones properties

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. 

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. 

Table 3. Properties of Diamond and Synthetic Gemstone Materials ...

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. 

Natural resins have been collected by hand throughout recorded history and used with minimal processing. They are reported to have been used in the arts, both in paints and for polishing sculptures, as early as 350 BC. Amber, the hardest of these resins, has been used as a gemstone from early Greek history to modem times. The electrical properties of amber were first recorded about 300 BC. Following is a description of commercial natural resins that are available in the United States. 

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... 

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. 

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. 

In a perfect crystal, all atoms would be on their correct lattice positions in the structure. This situation can only exist at the absolute zero of temperature, 0 K. Above 0 K, defects occur in the structure. These defects may be extended defects such as dislocations. The strength of a material depends very much on the presence (or absence) of extended defects, such as dislocations and grain boundaries, but the discussion of this type of phenomenon lies very much in the realm of materials science and will not be discussed in this book. Defects can also occur at isolated atomic positions these are known as point defects, and can be due to the presence of a foreign atom at a particular site or to a vacancy where normally one would expect an atom. Point defects can have significant effects on the chemical and physical properties of the solid. The beautiful colours of many gemstones are due to impurity atoms in the crystal structure. Ionic solids are able to conduct electricity by a mechanism which is due to the movement of fo/ 5 through vacant ion sites within the lattice. (This is in contrast to the electronic conductivity that we explored in the previous chapter, which depends on the movement of electrons.)... 

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. 

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