Colors of gems

The beautiful pure color of gems, so valued by cultures everywhere, arises from trace transition metal ion impurities in minerals that would otherwise be colorless. For example, the stunning red of a ruby, the most valuable of all gemstones, is caused by... 

Platonov A, Taran M, Balizkii B (1984) The nature of color of gems. Nedra, Moscow (in Russian) 196p... 

NMR spectroscopy is a convenient method for structural study of the equilibrium between the colored and the colorless form of spirobenzopyran. In the H-NMR spectra, the chemical shifts of gem methyl groups in 3 -position, (V-methyl group,2,11 and methine protons in 3- and 4-position are important to distinguish between the colored and colorless forms. 

Throughout history civilization has treasured the rarity and beauty of fancy colored diamonds. The stunning diamond from India known as the Hope Diamond, once a part of many royal inventories, is now the premier attraction of the Smithsonian Institution (see color Fig. 4.3.1). While the size of the diamond at 45.52 carats has certainly contributed to the public s interest in the gem, the intense blue-violet color of the stone is generally considered to be its most captivating feature. First described in the mid 1600s by the French merchant traveller Jean Baptiste Tavernier as un beau violet (a beautiful violet), the gem also acquired the title Blue Diamond of the Crown or the Royal French Blue when in possession of King Louis XIV of France. The blue color is attributed to trace amounts of boron in the carbon matrix of the stone. Substitution of carbon atoms by nitrogen leads to yellow diamonds, such as the famous canary yellow 128.51-carat Tiffany diamond. 

Chemical treatments are commonly applied to imperfect gems to improve appearance and/or enhance the color. Although gem treatment is legal, dealers must disclose whether or not a specific specimen has been treated and what treatment has been applied. As treatment technology improves, the gem industry is increasingly aware of and sensitive to the presence of both disclosed and undisclosed treated gems in the marketplace. Thus, there is a need to be able to reliably detect chemically treated gems with a minimally-destructive technique. 

The red color of the ruby is also caused by the presence in it of a trace of chromic oxide, which distinguishes this costly gem from common crystalline corundum (alumina). Thus chromic oxide, according to F. H. Pough, is the most valuable commodity in the world when purchased in the form of a ruby (84). A beautifully illustrated article on synthetic rubies appeared in the Journal of Chemical Education for June, 1931 (85). 

It is just this art of staining crystal which is represented very fully in the Swedish papyrus. There is no reference to colored glass gems as manufactured by the glass workers. This manuscript gives us the detailed explanations which make Pliny s statement more intelligible. 

The beauty of a gem is measured in terms of its clarity, brilliance, and color. Its natural beauty can be enhanced by the way it is cut. There are two basic kinds of gem cuts faceted and cabochon. The faceted cut has many flat cut surfaces (facets) with an overall shape that might be round, oval, square, rectangular, or pear-shaped. Faceted cuts are preferred for brilliant transparent stones such as diamond. The cabochon cut has a smooth rounded top, usually with a flat base, and it is mainly used for opaque or translucent stones. 

Long reaction times lead to the disappearance of the blue color of thione 1, related to the addition of a second molecule of hydrogen sulfide to give colorless gem-dithiols 3. Aromatic derivatives (R = Ar) are less prone to further addition of hydrogen sulfide, whereas with aliphatic derivatives (R = alkyl), gem-dithiols are generally the final reaction products and can be isolated and fully characterized.18 gem-Dithiols 3 can be converted into thioacylsilanes 1 by neutralization of the thionation solution with solid sodium hydrogen carbonate. With this procedure, enolizable acylsilanes 2 (R = R CH2) are stereoselectively transformed into Z-a-silyl enethiols 4 (vide infra). 

Because of its high abundance, iron is often found as an impurity in other materials. For example, corundum (y-Al203) of gem quality is sapphire, and its colors are caused by small amounts of FeIv. 

Figure 6 Cosmic jewels an aggregate of tiny blue hibonite crystals from the Murray CM chondrite. This entire object is no more than 100 xm in maximum size. The color of these sapphire-like gems is real and is due to the presence of substantial titanium in the crystal structure. The color variation is partly due to the crystals being compositionaUy zoned (their margins are more titanium-rich), and also to their being pleochroic under plane polarized light the color is visible only when the crystals are in specihc orientations relative to the polarizers on the...

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