Causes of Color in Gemstones




Earlier in this assignment, it was made clear that the cause of color in transparent gemstones is selective absorption, but little was said about the CAUSE of differential absorption and transmission of the various wavelengths. It is obvious that gemstones such as corundum and tourmaline, both of which occur in colorless forms as well as in numerous colors, must owe their color absorption to something that is not found in every stone of the species. Gem species of this kind have been shown by very careful chemical analyses to be colorless when absolutely pure. The various colors in which they occur vary according to the percentages of metallic oxides that are present in minute quantities as impurities. Such gem minerals are called ALLOCHROMATIC (pronounced al-oh-kro-MAT-ik). Those in which an essential chemical element is responsible for the color are said to be IDIOCHROMATIC (pronounced id-ee-oh-kro-MAT-ik). Examples of the latter are turquoise, peridot and malachite. Therefore, all specimens of an idiochromatic species are quite similar in color.

Prior to the introduction of the synthetic gem materials, causes of color were theorized on the basis of very accurate chemical analyses and by the results of the addition of various quantities of different substances to glass melts. Careful analyses of blue sapphire showed traces of iron and titanium, whereas ruby showed small amounts of chromic oxide. (Note: It is interesting that the chromium atoms replace aluminum in the corundum structure readily, which accounts for the fact that rubies are usually fairly evenly colored. On the other hand, the iron and titanium oxide that colors sapphire is present in a combined state in the form of the mineral ilmenite (FeTiO3), which is in a colloidal form. This may be the reason that sapphires are usually somewhat unevenly colored and that synthetic sapphires of a fine blue color have not been made). Since these were the only differences detected, the conclusion was inescapable. The Verneuil process for the synthesis of corundum confirmed suspicions along these lines, for it was found that oxides of various metals produced the desired and expected results. Subsequently, many different metallic oxides were tried, with the result that some colors were produced that are not known in nature.

As you have learned, synthetic gem materials duplicate their natural counterparts in both composition and structure; as a result, their properties are substantially identical. When speaking of corundum and synthetic corundum, the term "identical" applies without qualification with respect to structure, properties and composition. Synthetic spinel is not exactly the same in composition; consequently, its properties differ somewhat, but the structure of the natural and synthetic are the same. Synthetic spinel was first made accidently when magnesium was added to aluminum oxide in an effort to make blue synthetic sapphire. Fine blue in the synthetic sapphire had proved impossible to make, so many experiments were tried to improve the situation, without success. Even today, dark-blue synthetic spinel is much closer in appearance to top-quality blue sapphire than is any synthetic sapphire.

Synthetic emerald (beryl) has been made successfully in commercial sizes for many years. It was first made in Germany in 1935. Those now on the market are manufactured in San Francisco by Carroll Chatham, a chemist. Quartz, which also possesses a simple chemical composition, is made synthetically, but it is inexpensive enough in its more usual varieties to make its manufacture commercially unprofitable for gem purposes. Diamonds are made synthetically in minute sizes (several thousand pieces per carat) for industrial purposes by the General Electric Company.

Identical reproduction of the chemical composition of a genuine stone does not constitute synthetic reproduction of the genuine, unless the reproduction also has the same arrangement of atoms in its crystal structure and the same physical and optical properties. Manufactured "copies" of gems that have the same chemical composition and color as the genuine, but that do not reproduce their crystal structure, are glass, and are classified as imitations. For instance, a certain emerald-green material made of BeAl2(SiO3)6, which is the formula for genuine beryl, is not a synthetic emerald but an imitation, since it does not also have the same atomic structure as the genuine. Lacking the same structure as genuine emerald, it has different physical and optical properties. Since it has an amorphous structure, it is merely a glass imitation, even though it does passes the same chemical composition as emerald.

The color of synthetic corundum and synthetic spinel is produced by the addition of a small amount of a metallic oxide (a combination of a metallic element and oxygen) to the powdered compound of the species before it is fused. Different oxides are used to produce various colors. The green color of synthetic emerald is produced by the addition of chromic oxide.

Glass is actually more a physical state than a chemically definable material. It may be thought of as a super cooled liquid, so viscous that, for all practical purposes, it is rigid. It may be made with a wide variety of constituents, although the key ingredients are metallic oxides. Almost always a major constituent is silica (silicon oxide) in the form of pure sand. Oxides of sodium and calcium are usual constituents as well. Lead, boron or potassium oxides are also used frequently, but the composition depends on the properties desired. Most fine tableware and many gem imitations have a large lead-oxide content. Small amounts of other metallic oxides act as colorants; e.g., cobalt for blue.