When a Ray of White light passes through a coloured stone, light of some particular wave lengths is absorbed more strongly than others, the colours that are not absorbed – produce the colour of the stone.
If we use a Spectroscope and the different colours passing through the Stone will present a spectrum – this is different for different Gemstones and is called Absorption Spectrum.
Absorption Spectrum is a very important tool in identification of Gemstones and Spectroscope is the tool that helps us see this Spectrum.