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Color stability in fancy sapphire – what is this all about?

Color stability in fancy sapphire – what is this all about?

 

Even though color stability of fancy color sapphire has been discussed for decades, the issue has become more widely known only very recently, with more and more labs adopting rigorous color stability testing procedures, mostly as a result of the recently increasing popularity of padparadscha sapphire. Having received several emails from concerned clients within only a couple of days, we thought we should share some information.


This 5 ct unheated padparadscha has been positively tested for color stability by Lotus Gemology, Bangkok.

This 5 ct unheated padparadscha has been positively tested for color stability by Lotus Gemology, Bangkok.


The following is in no way meant as a scientific treatise on the issue of color stability (please refer to the big labs for academic writing on this…), but more as a practical FAQ for gem enthusiasts.

What is “unstable color” in sapphire?

Scientists rather speak of a gem having a “color center” that has two different states, a “relaxed” (inactive) and an “active” one. As an example, imagine a peach sapphire, with a basic orange color and a color center containing pink. If that color center is active, it will give the sapphire its overall peachy, if not even padparadscha-like color. If the color center is relaxed (inactive), the sapphire will only appear orange (but not peach-colored anymore).


These three unheated padparadscha sapphires have been positively tested for color stability by AGL (New York) and Gübelin (Lucerne, Switzerland).

These three unheated padparadscha sapphires have been positively tested for color stability by AGL (New York) and Gübelin (Lucerne, Switzerland).


What makes the color center become active or relaxed/inactive?

Color centers can become active or relaxed as a consequence of exposure (or the lack of exposure) to certain light sources and possibly heat. UV radiation plays an important role in this, with color centers commonly switching to their inactive/relaxed states when the gem is not exposed to UV radiation over an extended period of time. This can be the case, for instance, when a gem is stored in a safe for several months or even years. The phenomenon of “fading” (a color center becoming inactive) under such circumstances is well known for yellow sapphire, and similar effects have recently been observed in padparadscha – and other fancy color – sapphire. 

Is this effect reversible? 

Yes. The good news about this color stability issue is that an inactive/relaxed color center can always be activated again by exposure to, for instance, natural daylight. If a color center in a sapphire, for instance after years in a bank safe, has turned to its relaxed state, this does not mean that the stone is “ruined”. Under the right circumstances the color center can become active again. 

Does this have anything to do with the gem being treated? 

No. Unstable color centers have been detected in both heat treated and unheated/untreated sapphires. However, with heated (let alone beryllium-heated) padparadscha sapphires generally not being considered “the real deal” anyways, the issue is more relevant for unheated gems. 

How about origin? 

Unstable color centers can occur in sapphires from all geographic origins. However, our experience with numerous padparadscha sapphires has shown that those from Sri Lanka tend to have stable color (i.e. they almost without exception pass the color stability test, for instance applied by AGL). When we recently encountered a padparadscha that had failed the color stability test, it was in most cases from Madagascar. This is purely an observation, it does not mean that Madagascar is the only origin that produces sapphire with non-stable color. And of course it does not mean that all padparadscha sapphires from Madagascar have an unstable color center (some are perfectly stable, and this can be proven with a color stability test). 


This padparadscha sapphire is from Madagascar, but has been positively tested for color stability by AGL (New York).

This padparadscha sapphire is from Madagascar, but has been positively tested for color stability by AGL (New York).


Are all colors of sapphire affected?

No. Affected are sapphires in the color range of near-colorless, yellow, pink, lavender, orange, and padparadscha (orange-pink or pink-orange). As it seems, blue sapphires are never affected.

This near-colorless sapphire was diagnosed by AGL (New York) with a rare phenomenon. When the color center is active, it appears pale yellow-orange. When the color center is relaxed, it is near colorless with a slight bluish tinge. A collector was thrilled to acquire this rare gem from us, so an unstable color center is not always a bad thing.

This near-colorless sapphire was diagnosed by AGL (New York) with a rare phenomenon. When the color center is active, it appears pale yellow-orange. When the color center is relaxed, it is near colorless with a slight bluish tinge. A collector was thrilled to acquire this rare gem from us, so an unstable color center is not always a bad thing.


The AGL report for the colorless to yellow-orange sapphire mentioned above. Note the comments that AGL made about this stone.

The AGL report for the colorless to yellow-orange sapphire mentioned above. Note the comments that AGL made about this stone.


If the color of my sapphire is not stable, what does this mean for value? 

If your sapphire tests to have an unstable color center, there is no reason to be overly concerned if it is, for instance, a yellow sapphire. The phenomenon has been known for a long time to occur in yellow sapphire, and due to its reversible nature, it has little effect on value. 

This is different when it comes to padparadscha sapphire. So why is that? Actually, several labs, including AGL and Gübelin, have stopped granting the “padparadscha” color designation if the required combination of pink and orange is not visible in both the active and the inactive state of the color center. This means: a sapphire that looks like a padparadscha with the color center active, but not anymore with the color center inactive/relaxed (or vice versa), is NOT CONSIDERED A PAD ANYMORE by most major labs. As a consequence, countless sapphires that would have been considered a padparadscha less than a decade ago, will not be called padparadscha anymore by the major labs. The consequences for the value of an affected gemstone are dramatic. 


Does this mean a sapphire with an unstable color center can never be a padparadscha? 

No. If a sapphire shows an unstable color center, but both states (color center active/relaxed) show a padparadscha-like combination of orange and pink, the major labs – such as AGL and Gübelin – will still consider it a padparadscha. Why? Because the desirable color is always there, it only shifts around a little. In some cases, an orange-pink sapphire can shift to pink-orange, and both combinations are unquestionable pad colors, so no issue there.


AGL report for our 1.05 ct Madagascar padparadscha. Please note AGL’s comment “stable color identified” – when shopping for padparadscha, make sure this is what you get (and pay only very little for a non-stable pad-like sapphire).

AGL report for our 1.05 ct Madagascar padparadscha. Please note AGL’s comment “stable color identified” – when shopping for padparadscha, make sure this is what you get (and pay only very little for a non-stable pad-like sapphire).


How can I be on the safe side when buying sapphire?

If you want to be sure that the color of your sapphire is stable – which is relevant mostly when buying padparadscha – buy only with a recent lab report that is stating both that a color stability test has been performed, and that a stable color has been identified. AGL, among other labs, is offering reports like that.

 
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