Is it a Padparadscha, or not?
[This feature has been updated: December 20,2022]
GIA Observes Unusual Color Change in Sapphires During Color Stability Testing
… and what is Tenebrescence?
Above – Image copyright GIA, Press Release
Gary Roskin –
Roskin Gem News Report –
December 14, 2022 –
Over the past several years (2018/2019 till today), more and more sapphires have undergone color stability testing as gem dealers discover sapphires with color instability … elementary, they change from pink to pink-orange (padparadscha), and then back again to pink.
This color change is NOT due to changing light sources e.g. incandescent to fluorescent and seeing an alexandrite-like color change. No, this is caused by exposure to ultraviolet light (where it gains yellow and/or orange) and then reverts back by heat [not heat treatment], lack of light, or strong incandescent light. This is called Tenebrescence.
Mindat.org defines Tenebrescence as: the ability of minerals to change colour when exposed to sunlight. Tenebrescent minerals include hackmanite, spodumene and tugtupite. Tenebrescent behavior is exploited in synthetic materials for the manufacture of self-adjusting sunglasses, which darken on exposure to sunlight.
Mindat gives examples of hackmanite (variety of sodalite), spodumene and tugtupite. You will note, sapphire (corundum) is not listed. While it’s been known for decades that some sapphires can have this property, stability testing has only been a standard lab procedure for yellow sapphires. Not until recently has stability testing been performed on pink and/or padparadscha (pink-orange).
Typically, the pink/orange sapphires with unstable color would be pink in its stable position, then change color to a padparadscha pink-orange under strong UV, and then with strong incandescent light, revert back to pink. You can also revert the color back to pink by heat (150°–300°C), exposure to intense white LED light, or leave it in the dark for an extended period of time.
But here is the new issue. According to GIA, “A padparadscha sapphire (3.54 cts.) submitted to the GIA laboratory increased its orange color during the color stability test [exposure to strong incandescent light]. If left in the dark for an extended period, the color could become less orange or even go to straight pink. This can be replicated by exposing the sapphire to intense LED illumination.” [note: LED, not incandescent light.]
And here is the conclusion: “This stone would be characterized as a padparadscha sapphire because the pink-orange color after the stability test is acceptable.”
So with this very unusual 3.54 carat sapphire, instead of the sapphire exhibiting the usual tenebrescent results by reverting back to pink when exposed to strong incandescent light, the sapphires gained orange color to end up as a pinkish-orange padparadscha color, and sometimes even to orange!
Of course, this causes some concern about how to label the color of these sapphires. Are they then Padparadschas?
According to GIA’s press release, GIA states that, “In GIA’s experiments, the color observed after the color stability test is very similar to the color observed after exposure to daylight, for example, if set in jewelry and worn frequently. Given this and the need for a consistent, easily accessible standard, the color after the incandescent light stability test is the color indicated by GIA on reports for these sapphires. This standard is applied regardless of whether a sapphire loses or gains yellow or orange color during the test.”
You can read more about this discovery in a recently published feature in a GIA Lab Note, “An Update on Sapphires with Unstable Color,” by GIA gemologists Dr. Aaron C. Palke, Shane F. McClure and Nathan R. Renfro.
An Update on Sapphires with Unstable Color
Aaron C. Palke, Shane F. McClure, and Nathan R. Renfro
December 12, 2022
