A spectroscope will show you the wavelengths of light emitted, or reflected, from a gem. The absorbed wavelengths of light, on the other hand, will appear as a darker shade of color or black. Since some absorption patterns are distinctive, they prove useful for gem identification.
The spectroscope is also a great asset for examining rough gems, when getting RI readings is difficult, and detecting some treatments. In some cases, only a spectroscope can detect if a gem’s color is natural, dyed, or irradiated.