Gemstones physical properties

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. 

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.)... 

The manner in which atoms are bound together in a given substance has a profound effect on its chemical and physical properties. For example, graphite is a soft, slippery material used as a lubricant in locks, and diamond is one of the hardest materials known, valuable both as a gemstone and in industrial cutting tools. Why do these materials, both composed solely of carbon atoms, have such different properties The answer, as we will see, lies in the bonding within these substances. 

Diamond is the hardest known substance, and apart from its commercial value as a gemstone, it has apphcations in cutting tools and abrasives (see Box 13.5). The structural differences between diamond and graphite lead to remarkable differences in physical properties (see Section 13.3) and uses. The properties of graphite that are exploited... 

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. 

A conventional listing of the more important nonmetallics is presented in Table 12.2. Gemstones. A gemstone is a semi-precious or precious natural mineral with exceptional physical properties which, when cut and polished, can be used in jewelry. Only four minerals are considered as precious gemstones sensu stricto diamond, one gem variety of beryl (i.e., emerald green), and the two gem varieties of corundum (i.e., ruby, deep red, and sapphire deep blue). Beside natural minerals synthetic gemstones and their simulants are also found in jewelry. 

Klein, Cornelis, and Barbara Dutrow. Manual of Mineral Science. Hoboken, N.J. John Wiley Sons, 2008. Describes how to identify minerals using physical properties and modern analytical techniques. It has a chapter on 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. 

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