Photos by Diego Sanchez
Determining and reporting the colour of some sapphires continues to pose a unique challenge for gem scientists and jewellers alike.
This is among the key takeaways of a recently published Lab Note by the Gemological Institute of America (GIA). During routine colour stability testing of sapphires, researchers at the laboratory observed an unusual gain of orange or yellow colouration to a padparadscha-like or pure orange colour.
Colour in sapphires can be unstable, GIA reports. As such, the laboratory routinely tests the gems for stability by exposing them to intense incandescent illumination for a few hours. The test causes 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 colouration,” GIA said in a statement. “Some pink sapphires might shift to a padparadscha colour or pure orange.”
The observed behaviour raises questions about determining and reporting the colour of sapphires exhibiting this change. In the institute’s experiments, colour observed after a stability test is very similar to that which is observed after exposure to daylight (i.e. when a stone is 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.
Preliminary observations and research on these findings have been published in the GIA lab note, “An Update on Sapphires with Unstable Color” by Aaron C. Palke, PhD, Shane F. McClure, and Nathan R. Renfro.
