Zircon that changes from blue to brown when exposed to long-wave ultraviolet light

good morning.

This is Nori-chan, a GTJ staff member.

I also want to change and evolve.

In early October 2024, we suddenly introduced several zircons that appeared to be a mixture of bright blue and green.

At first, he said, "I bought it because I was attracted by its bright, mysterious and beautiful color." However, he discovered that it underwent an interesting change when exposed to UV light during photography, a process that is carried out before it is sold as a product. This is the change in the color of the base, which is the subject of this article, and is commonly known as the "tenebressence effect."

Tenebrescence (photochromism) is an optical effect in which the color of a mineral changes as it absorbs long-wave ultraviolet light. A typical example is hackmanite, which absorbs long-wave ultraviolet light and turns deep purple, then gradually returns to its original color. (※1)

This article is about zircon, which has interesting changes, and is not about specialized content such as mineralogy, but rather about observing minerals, like an experiment that anyone can do if they have the equipment.

This zircon was one of several introduced during the live show. The zircon's body color is a pale brownish blue. (※2)

UV light is applied. It is irradiated for about 1 minute during live performances. This UV light is designed to block visible light, so you can feel a significant change even after 10 seconds.

After about a minute of exposure to long-wave UV rays, the color has turned dark brown. The blue has disappeared quite clearly, and the brown has become more prominent.

It feels like the zircon, which is brownish blue (light blue) with a metallic luster, has changed color to a very solid brown zircon. The color change is more noticeable than I expected, but because the original color contains a blue tint, the difference in color change before and after is more clearly visible.

There is currently no mention of "tenebressence" in the EGL sorting. However, since there have been so many changes, I took the opportunity to request an identification certificate from the Japan-Germany Gemological Institute.



The image above is the test result. In the remarks column, it says "Tenebrescence (photochromism) observed." Pieces with too little change are not recognized as tenebresence, but I think that this lot has changed to a degree that is recognized as tenebresence. (※3)


This raises a question: "Will other blue zircons change color?" I decided to try exposing some blue zircons I had on hand to long-wave ultraviolet light.
This is a vintage zircon with a slightly stronger blue color. Since it is blue, it is a piece made by heating Cambodian zircon at 800-1000℃.
If you expose it to long-wave UV light for about 30 seconds...




It's hard to say that it has changed, but I was able to sense a slight difference in color. It feels like it has absorbed long-wave ultraviolet light and become slightly brownish. However, at this level, it might be more accurate to say that it has "become duller than changed." Even if I took it to the Japan-Germany Gemological Institute, there is a high chance that it would not be recognized as tenebresence. I have seven blue zircons on hand, and I was able to observe some kind of change in all but one.
Generally, the color just becomes dull, but the zircon we introduced has a large color change from blue to brown, which may be a little unusual.
Currently, there are only a few commercially available zircons that show the clear color change that can be called the tenebrescence effect, but it seems likely that more interesting zircons will be discovered in the future.

*1 This time we will not explain the principle, but only the visual changes.
*2 This zircon is a privately owned reference stone and is not for sale.
*3 When identifying such minerals, the Japan-Germany Gemological Institute will also add photos if you ask them to "check whether tenebresence is present, and if so, to provide photos of the before and after changes." (Additional charges apply.)