IDT Gemstones

All colours of corundum excepts for red and blue are called fancy-coloured sapphires. These fancy-coloured stones should be described by colour, for example, ‘green sapphire’, ‘yellow sapphire’ and ‘purple sapphire’. The only exception to this is the rare pinkish-orange stones called padarsdscha.

Common Treatments

The irradiation of colourless to pale yellow sapphire produces a deep yellow to brown colour which can fade rapidly on exposure to heat or light. Treated yellow sapphire is difficult to distinguish from natural yellow sapphire.

Tests

Dichroscope- Strong in varieties other than yellow. Other colours will show differing shades of the body colour. Corundum is dichroic.
UV — Almost colourless, yellow and golden sapphires from Sri Lanka usually glow apricot or orange under LWUV light; they do not usually show an iron spectrum, which distinguishes them from similar-coloured stones found in Australia which do not fluoresce but show a typical sapphire iron spectrum.
Spectrum— Green and golden stones can have similar spectra to blue sapphires.

Quality Factors

For fancy coloured sapphires, the factors that will influence the value of the stone are the depth of colour, clarity and appearance. However, another important factor is the colour itself, for example these five fancy sapphires have been placed in value order according to one UK stone dealer. The natural coloured padaradscha on the left being the most expensive. The yellow, although a nice bright stone, is the least as it is a more common colour than some of the others in this group, so has a lower price per carat. The effect of colour on price will also be dependent on fashion and local demand.