During routine colour stability testing of pink sapphires, GIA experts observed an unusual gain of orange or yellow coloration to a padparadscha-like colour or even to pure orange.
Colour in sapphires can be unstable, which is why GIA routinely tests them, including those that may be characterised with padparadscha colour, for stability by exposing them to intense incandescent illumination for a few hours. This test will cause an unstable orange component to fade, returning the stone to pink. In recently observed tests, some sapphires reacted to the colour stability test by gaining orange or yellow coloration. Some pink sapphires might shift to a padparadscha colour or pure orange.
This colour change behaviour raises important questions about determining and reporting the colour of sapphires exhibiting this colour change. In GIA’s experiments, the colour observed after the colour stability test is very similar to the colour observed after exposure to daylight, for example, if set in jewellery and worn frequently. Given this and the need for a consistent, easily accessible standard, the colour after the incandescent light stability test is the colour indicated by GIA on reports for these sapphires. This standard is applied regardless of whether a sapphire loses or gains yellow or orange colour during the test.
GIA’s preliminary observations and research into this important matter were recently published in a GIA Lab Note, An Update on Sapphires with Unstable Color, by GIA gemmologists Dr. Aaron C. Palke, Shane F. McClure and Nathan R. Renfro. The full results of GIA’s research into this matter are currently being prepared for publication in Gems & Gemology, GIA’s quarterly academic journal, in the near future.