Gems have the ability to separate “white light” (the mixture of all colors) into its various colors, producing flashes of red, yellow, green, and other colors. Separation occurs because the various colors, or wavelengths composing white light passing through the gem, are each bent or refracted a different amount. Red is bent least, followed in order by orange, yellow, green, blue, and violet, which is bent most. This characteristic of being able to produce flashes of color, as seen prominently in diamond, is known as _dispersion_ or _fire_. Quartz and glass have low dispersion, and hence they make poor diamond substitutes. Some of the newer synthetic gemstones, such as titania, have extremely high dispersion, with resulting fire. Zircon, a natural gemstone of suitable hardness, exhibits high dispersion and is a commonly used substitute for diamond. CHEMICAL CHARACTERISTICS OF GEMSTONES Since gems are embraced in the mineral kingdom, and minerals are naturally occurring chemical substances, it follows that all the accepted terms of chemical description can be applied to them. When a chemist learns that ruby is an impure aluminum oxide, he understands a great deal about the nature, origin, and behavior of ruby. He can assign to it the chemical formula Al₂O₃, symbolizing its basic composition as two atoms of aluminum united with three of oxygen. Similarly, other popular gemstones can be described chemically as follows: Diamond Carbon C Sapphire Aluminum oxide Al₂O₃ Quartz Silicon dioxide SiO₂ Emerald Beryllium aluminum silicate Be₃Al₂(SiO₃)₆ Spinel Magnesium aluminate Mg(AlO₂)₂ Significantly, ruby and sapphire are chemically identical, both being of the mineral species corundum. As already explained, the difference in color is due entirely to very slight traces of chemical impurities. Frequently, the impurities are present in irregular patches that give spotty color effects. Some mineral species possess many of the desirable qualities of gemstones yet cannot be used as gems because they are chemically active and therefore are less durable. They undergo alteration and decomposition when exposed to light or to one or another of such substances as air, water, skin acids and oils. 3 THE SHAPING OF GEMSTONES Gemstone crystals often have naturally brilliant, reflecting faces, but rarely are they perfect and unblemished. Also, their natural shapes do not provide the best expression of their luster, brilliance, dispersion, color, and other inherent properties. In fashioning a gemstone, the skilled artisan tries to develop these hidden assets and to otherwise enhance the gemstone’s general beauty.