Spectroscopes are used to generate and analyse absorption spectra of coloured stones. These absorption spectra are almost like individual fingerprints which allow many stones to be determined or stones of the same colour to be separated quickly and reliably. Transparent or translucent stones in rough or polished condition are suitable for this type of analysis. It may also be possible to observe the spectra of opaque stones if the light reflected from the surface is used for this purpose. In the case gemstones of different colours, various absorptions take place, for which the respective individual chemical compositions and crystal structures are responsible. These absorptions determine the chromaticity of the stones at the same time. By nature, only clearly coloured stones produce an absorption spectrum, since no absorption takes place with colourless stones and pale coloured stones do not absorb sufficiently for a clearly visible spectrum.
The spectroscope has a narrow slit for for light entry or light incidence, in front of which a stone is placed. If the stone is illuminated by light, a certain part of the light is absorbed by the stone. The other part is broken down into its absorption spectrum inside the spectroscope and is visible to the observer in the eyepiece.
Dark lines and stripes appear where certain wavelengths are absorbed by the stone. This creates an absorption spectrum that is visible to the observer in the eyepiece. By fading in the wavelength scale, absorption lines can be measured directly. Reference spectra for the most common gemstones are listed and illustrated in the gemmological literature. An example are the Fraunhofer lines and absorption bands (right figure).
Do you want to read more?
If you are looking for more specialist information, order the white paper “Gemstone Spectroscopy”. The publication describes e.g. the following topics:
- Design and functioning of a spectroscope
- Spectral colours, produced by refraction of white light
- The visible spectrum for humans
- Comparison of the absorption spectra of red gemstones
- How do spectra of colours come into existence?
- Practical tips: Spectroscope examination
- Extract of colour chart Fraunhofer’s lines and absorption bands
- Spectra of ruby, red spinel, garnet, almandine, alexandrite, red tourmaline
The whitepaper was produced in collaboration with Prof. Dr. Jochen Schlueter wrote. He is a graduate mineralogist and responsible for the display collection of the Mineralogical Museum in Hamburg.
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