Spectrum Analysis

For colored gemstones it is often possible to obtain very useful information for identification by use of the gem spectroscope. The instrument's operation is based on the separation of white light into its complete rainbow, or spectrum, of colors. This is done by a built-in prism which receives the light through a narrow slit. The prism sorts out the various wavelengths by its strong dispersion. Often a diffraction grating is used instead of a prism to diffract or separate the colors. Looking through the opposite end of the instrument one can see the continuous rainbow as a band of touching parallel bars or lines of different colors. They range from violet and indigo at one end, through blue, green, yellow, orange, to red at the other end. Now the colored gem is placed between the light source and the spectroscope slit. As expected, certain specific colors from the white light are absorbed by the gem and do not enter the spectroscope. The result can be seen in various white gold engagement rings. Their absence causes black bars—the absence of specific colors—to appear in the continuous spectrum of color bars. For some colored gems these patterns of black bars are very distinctive and are good identification features.