How The Spectroscope Works

The spectroscope splits visible light into its component colors (using either a set of prisms, or in the less expensive models, a diffraction grating) producing essentially the colors of the rainbow--red, orange, yellow, green, blue and violet. A diffraction grating spectroscope achieves this by using a finely ruled grating consisting of a series of photographically etched parallel lines (as many as 1000 per millimeter) on a sheet of glass. This produces a series of spectra by diffraction and interference, but only the strongest spectrum is used. For proper viewing of gemstone spectra a concentrated light source is required.

Lighting and Use
With an adjustable slit spectroscope, resist the temptation to open the slit wider than it necessary as it only tends to make matters worse by drowning the spectrum with too much light. Instead, try moving the stone to let more light through it, or use a lens to further concetrate light into the stone.

The difficulty in reading lines, bands or absorption in the blue violet end of the spectrum can be overcome, in some instances, by filtering the incident white light through a flask containing a solution of copper sulphate and distilled water (copper sulphate is toxic, so care is necessary when handling it). By filtering out the distracting longer wavelengths (red, orange, yellow, etc) the eyes adjust more readily to the subtle differences seen in the blue violet end.

Inability to see a spectrum may be due to the fact that only a small trace of the spectrum causing agent is present, or that insufficient light is reaching the spectroscope for analysis (e.g , in very dark stones or semi-translucent to opaque material). In very pale or shallow stones, try passing the light through the maximum amount of material in order to enhance the strength of the absorption pattern seen.

Keep in mind
Not every stone has a spectrum, and not every spectrum you see is helpful in gem identification. More important--don't expect all spectra to look as neat and tidy as those illustrated in books. In stones with different selective absorption (dichroic or trichroic) the spectrum may vary depending upon the direction in which the stone is viewed. Spectrum analysis is only one test leading to positive identification of a stone.