A gemological instrument that separates a ray of light into its spectral colors by means of prisms or a diffraction grating. It is used for observing the spectra of gemstones. Light passing through or reflected by a gemstone into a spectroscope yields a spectrum for that gem. The spectrum that results from a spectroscopic examination may not definitively identify the gem, but the lack of the “usual” spectrum for the gem under examination makes the identification of that stone highly suspect. This is one of many tools a geologist uses to identify gemstones.