Natural sapphires are completely colourless. The presence of trace chemicals provides colour through the natural process of heat and pressure. The easiest way to improve clarity and colour of sapphires is to mimic the natural process and apply heat via an oven or furnace. Heating a sapphire at high temperatures specifically dissolves inclusions known as "silk" and alter other impurities, making the gemstone more valuable. Heat treatment is the most desirable gemstone enhancement because it does not use chemicals to alter the stone. Standard Heat-treating process is a common industry practice and accepted worldwide.
The Fourier-Transform Infrared Spectrophotometer (FTIR) is used to help determine whether a sapphire has been thermally enhanced. The heights of the various peaks give the gemmologist an indication of whether the sapphire being tested has been thermally enhanced.
For example, when a strong peak at 3160 wave numbers (cm-1) is found, this is an excellent indication that the sapphire being tested has not been thermally enhanced.When a strong peak is found at 3309 in non-basaltic sapphire, this gives an indication that the gem may have been heat treated. Interestingly, a weak-to-medium 3309 peak can be found in both heated and unheated sapphire. Thus great care must be given to interpreting FTIR data.
This Fourier-Transform Infrared Spectrophotometer is one of the most important tools in our arsenal. It is useful in helping to separate a number of natural gemstones from their treated counterparts, including various sapphires, emerald and jadeite, to name but a few.