Many gemstones can split a beam of light and bend one part more than the other, thus producing double refraction, or two different measurements of refractive index.
When a ray of ordinary white light enters some gemstones it is dispersed (split up) into rays of the separate colors of which it is composed. These rays are reflected inside the gem and are further separated by additional refraction as they leave the gemstone. This dispersion accounts for the colored flashes of light, or fire, for which diamond is highly prized.
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.