One of the most important phases of the study of colored stones concerns the optical properties of the various gem species, which permit accurate, safe and rapid identification of both mounted and un-mounted stones. Prior to the recent introduction of optical gem-testing instruments, testing procedures were limited mainly to destructive and very unreliable tests that constantly exposed the jeweler to criticism and even lawsuits. This is a major reason why the majority of jewelers have simply avoided handling colored stones, other than perhaps a limited number of standard, inexpensive stone-set rings. A knowledge of the properties of colored stones and of identification procedures constitute fundamentals that should be common knowledge to every jeweler.
A clear understanding of optical properties and their roles in identification and grading begins with a study of the nature of light and its behavior in the various gems. Optical properties, which represent the main source of a gem's beauty, may be defined as those properties that determine the effect a given substance has on light as it is transmitted by and/or reflected from it. The following discussion of the nature of light includes only the essential information that is needed in the study of gems and in the application of this knowledge to identification, grading and appraisal procedures.
THE NATURE OF LIGHT
Visible light, whether emitted by the sun or a candle or a light bulb, is a form of RADIANT ENERGY. The portion of the sun's radiant energy spectrum that we can see represents but a small fraction of the total; the remainder is invisible. The ELECTROMAGNETIC SPECTRUM, which includes infrared light, visible light, ultraviolet light, radio and television waves, X rays, cosmic rays and all other forms of radiant energy, is represented in Figure I. All of these have two properties in common: (1) they travel in air at substantially the same rate of speed (186,300 miles per second), and (2) they may be thought of as traveling in a wave motion. Physically, these forms of wave motion differ only in FREQUENCY (the number of vibrations, or cycles, per second) and in WAVELENGTH (the linear distance between corresponding positions on a wave). Figure 2 illustrates a wavelength. Although there is no distinct dividing line between the various forms of radiant energy, since they represent a continuous spectrum, there are extreme differences in the wavelengths of the various portions of the spectrum, Radio waves may be as much as six miles long, whereas, cosmic rays, at the other end of the spectrum, are only on the order of one-trillionth of a centimeter long! Since the velocity of all of this radiant energy in air may be regarded as equal, and the wavelengths differ so widely, the frequency of the short wavelengths have to be tremendous in comparison to that of the long wavelengths. In other words, the longer the wavelength, the lower the frequency; the shorter the wavelength, the higher the frequency.
The portion of the electromagnetic spectrum that is of primary concern to the gemologist is visible light. Visible light represents a spectrum ranging from the longest wavelength (red) through orange, yellow, green and blue, down to the shortest wavelength (Violet). When these wavelengths are mixed together, as we ordinarily see them, the result is WHITE light. The wavelengths of the visible spectrum range from approximately 7600 Angstrom units for red to approximately 4000 Angstrom units for violet. An Angstrom unit (abbreviated A.U.) is one ten-millionth of a millimeter (Figure 1). In order for violet light to travel as rapidly as red light, its frequency must be approximately twice that of red light. This may be likened to a man and a child walking for a distance of a mile side by side. If the man's stride is four feet and the child's are two, the child must take twice as many steps in order to maintain the same pace.
SOURCES OF LIGHT
There are two basic sources of light: INCANDESCENCE AND LUMINESCENCE. A solid mass may be an incandescent source of visible light, as when a metal is heated to a point where it begins to emit visible radiation. Thus we say that a metal is "red hot" or is at a "white heat". An incandescent light bulb contains as its principle element a tiny filament that is heated by electricity to a white - hot temperature. In contrast, luminescence does hot involve high temperatures.
Materials may be stimulated to emit visible light by a form of radiant energy such as ultraviolent light, which is invisible; this is called FLUORESCENCE. The typical fluorescent bulb used today for general illumination operates by an electrical discharge that gives off ultraviolet radiation. A fluorescent coating on the inside of the tube has the property of transforming the energy of the ultraviolet into a visible wavelength.
A number of gemstones (particularly opal, kunzite and diamond) fluoresce in various corers when subjected to ultraviolet radiation. If the reaction continues after the stimulating radiation is removed, the resulting light is called PHOSPHORESCENCE. Although there are other forms of luminescence, fluorescence is the most important type.