Colour

Gemstone colour arises when light is absorbed as it passes through the stone, but more importantly, a specific wavelength must be absorbed in order to give a certain colour. If, for example, all wavelengths are absorbed equally, the gemstone would appear grey or even black!

Figure 3.1 - Absorption Gives Rise to Colour

But why are certain wavelengths of light absorbed?

Pure corundum is completely colourless as there is no specific wavelength of light absorbed. If, however, tiny amounts of iron and titanium ions replace some of the aluminium cations in the corundum structure, a brilliant blue colour is observed (blue sapphires). Similarly, if chromium ions are present, the corundum takes on a vibrant red colour (rubies). Other trace metals give rise to different colourings.

Consider a tetravalent titanium cation (Ti⁴⁺) and a divalent iron cation (Fe²⁺), occupying nearby aluminium sites in the corundum structure. Note that the charges on the ions differ from that of aluminium (Al³⁺), but the overall charge is unchanged. If these ions are close enough to each other, an electron can be transferred from the iron to the titanium:

and the reverse process returns the system to its original state:

There is an energy difference between these two states, so a photon can be absorbed to provide the energy for the electron transfer. It just happens that for sapphires, the energy required can be gained from photons in the red region of the spectrum. Red light is therefore absorbed, leaving blue light to be transmitted. This gives sapphires their brilliant blue colours.

Dichroism

Sapphires belong to the trigonal (hexagonal) crystal system so, like calcite, exhibit birefringence. This means that a light beam passing through the crystals is split into two separate parallel beams, the ordinary and extraordinary rays. In addition to this, sapphires are dichromic, which means that the ordinary and extraordinary rays are slightly different colours:

The ordinary ray is a deep blue, where the extraordinary ray is a much paler yellowish-blue.