Colors of gemstones

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. 

The beautiful colors of gemstones such as rubies, emeralds, and sapphires, are due to fransifion-mefal ions presenf as minor componenfs in an ofherwise colorless mineral, such as quarfz or alumina. 

Small amounts of metals cause the different colors of 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. 

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. 

Ruby and Sapphire. Ruby and sapphire are "sister stones" both are gemstone forms of the mineral corundum (composed of aluminum oxide). Pure corundum is colorless, but a variety of trace elements cause corundum to exhibit different colors. Ruby is red corundum, while sapphire is corundum in all colors except red. The red in rubies is caused by trace amounts of chromium the more intense the red color of a ruby, the more chromium it contains. The blue in sapphires is caused by titanium and/or iron impurities (Garland 2002 Hughes 1997). 

Arem, J. E. (1987), Color Encyclopedia of Gemstones, Van Nostrand-Reinhold, New York. Armiento, G., D. Attanassio, and R. Platania (1997), Electron spin resonance study of... 

Diamonds are the only gemstone whose colorlessness enhances their value. However, the rare, rich, natural coloring of fancy color diamonds commands the highest prices. The Hope Diamond possesses exceptional blue coloring and is undoubtedly the most celebrated diamond in the world. What is the origin of its intensely prized blue hue ... 

In 1960 the first ruby laser was made from a ruby crystal of aluminum oxide (Al Oj). These crystals contain only a small amount of chromium, which stores the energy and is responsible for the laser action. A small amount of chromium found in the mineral corundum is responsible for the bright red color of the ruby gemstone. 

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. 

Color - [GENETIC ENGINEERING - PROCEDURES] (Vol 12) -of gemstones [GEMSTONES - GEMSTONE MATERIALS] (Vol 12)... 

Arum. J.E. Color Encyclopedia of Gemstones. 3rd Edition, Chapman Hall. New York. NY. 1994. 

A handful of a variety of gemstones. The beauty of gemstones is measured by their clarity, brilliance, and color. 

Tourmaline is a highly complicated silicate, with a wide range of compositions and colors. It probably exhibits more colors than any other kind of gemstone. Sometimes there are several different colors in the same crystal. Watermelon tourmaline, for example, is green on the outside but red in the middle. 

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