Chromium (steady-state) luminescence in gemstones

As its name indicates, the transition metal chromium contributes colour to our world. The purple/red of ruby and the vivid green of emerald both arise from traces of chromium within their crystal structure. Chromium oxide is a green pigment familiar to artists and chrome yellow was originally used for painting US school buses and German postal vans.

A property that is important for gemstone colouration is that the Cr^3+ ion and the Al^3+ ion have similar physical sizes and this means that chromium can occasionallly replace the aluminium in a crystal structure such as corundum (Al_2 O_3) at around the one percent level, resulting in ruby. The three outer electons remaining in the Cr^3+ ion are free to interact with visible light in a number of ways that each contribute to the colour of the crystal (the electrons are 'unpaired').

The role that these electrons play in the absorption and emission of light depends on the strength and symmetry of the electric field around the ion produced by the neighbouring atoms in the crystal: oxygen in the case of ruby. In crystals with different compositions such as the examples in this figure, the electric field - or 'ligand field' as it is known - is different. This moves the absorption bands to different wavelengths and produces different colours such as the rich green of emerald and the pink-red of spinel.

Some of the light energy that is absorbed by these crystals is converted directly to heat, but some fraction remains to be re-emitted as light in the far-red part of the spectrum. This luminescence - the conversion of shorter wavelength (bluer) light to red light - can produce rather pure colours (called narrow lines by spectroscopists), notably those in ruby that were utilised by T H Maiman in 1960 to produce the first laser. The symmetry and strength of the ligand field in the different gemstones affects the number and positions of these luminescence lines to produce the variations shown in the figure. Some of the stones shown here are synthetic but they display the same types of luminescence as the natural crystals. The luminescence patterns can be used with high confidence to identify the gemstone.

These observations were made by exciting the crystals with a 20mW 404nm blue diode laser, covering the spectrometer input with a yellow filter to exclude the scattered blue exciting light.

The narrow emission line in emerald at 683nm in these two examples is rather weak. I have another natural emerald - a small cut stone - where the line is relatively stronger by a factor of about 10.

Note from 11 May 2017: I realise now that the small triangular yellow stone referred to here as a topaz is actually a chrysoberyl ( Be Al2 O4). This has been confirmed with a measurement of the refractive indices as 1.73, 1.74.

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