Impact of Fluorescence on Diamond Color Grading
Everyone is used to the "ooohs" and "aaaahs" of seeing the amazing color changes of various gems and minerals when viewed under ultraviolet light. As we see in this calcite and Franklin specimen at left, the effects of fluorescent lighting on certain gems and minerals can be awe-inspiring.
Beyond the "ooohs" and "aaaahs," however, are some very important uses for fluorescence in gemology, along with a couple of places where this same fluorescence needs to be avoided.
Today, we want to take a tour of the good and bad impact that fluorescence has, particularly on diamond color grading. But first...let's look at some lab created emeralds and the impact seen on these.
When we view this large parcel of Biron created emeralds with a few Chatham Created Emeralds thrown into the mix, it is quite easy to utilize the fluorescent reaction of the Chatham Created Emeralds to separate them from the Biron. The reason for this is the Chatham Created Emeralds are virtually identical to naturally occurring emeralds, while the Biron emeralds are colored by large amounts of iron to impart the green color. Iron absorbs the energy of the fluorescent light while the more natural Chatham Created Emerald fluoresces brightly (as expected for both natural emeralds and true lab created emeralds.) The result is that the use of fluorescence can be a quick and easy separation tool for the Chatham Created Emeralds and the Biron created emerald.
Fluorescence in diamonds, however, is a totally different issue. Normally colorless diamonds that react strongly to fluorescent lighting can take on an unnatural color due to the reaction. Below you see a parcel of melee diamonds seen under a diamond color grading lamp, and under a long wave ultraviolet light (fluorescent light). Notice that some of the diamonds do not react, while others react very strongly.
Here, of course, is where the problem comes in regarding diamond color grading: many of the commercially sold diamond color grading lights use fluorescent light as their sources. As a result, what may appear to be controlled diamond color grading light will also have a varying degree of the ultraviolet light included in the light produced and used to grade the diamonds. The issue is this: If a diamond will react with a FL induced white color, or perhaps even blue color, in fluorescent light this can cause errors in diamond grading, both better and worse than the diamond color should be if the fluorescent light source is cleaned of the UV wavelengths.
The question is: How do you ensure you do not have ambient UV light causing errors in your diamond color grading? In other words, how do you make sure that other fluorescent light sources including sunlight coming through a window does not impact the true color grade of a diamond being evaluated?
The answer is at your local hardware store: Lexan®, a polysilicate material used in window panes that will block the UV light without interfering with the proper lighting wavelengths produced for diamond grading. Here is a demonstration below:
As you will note at left, when our sheet of Lexan is placed between the UV light source and our diamond parcel, the UV light is blocked. Moving from side to side this clearly shows that the Lexan will block the UV light from interacting with the diamonds.
When we place the Lexan totally in front of the UV light source, the UV light is totally blocked from our diamonds and is not allowed to impact the color of the diamonds based on the UV reaction.
Now, let's apply this demonstration to our diamond grading light and see what the reaction will be.
If you take just a few seconds and allow the slides at left to go back and forth you will see the impact of using Lexan on a commercial diamond grading light. The brighter slide is without the Lexan, the darker slide is with the Lexan in front of the diamond color grading light.
The ability to color grade the diamond is not affected, however the false color impact of the fluorescence of the light is blocked and a more true and accurate diamond color grade can be achieved.
It is important to remember that this is for diamond color grading, not testing a diamond for fluorescent reaction. The key is to block out all ambient fluorescent light from the color grading area to ensure that outside fluorescent light is not affecting the apparent diamond color grade.
In case you are curious as to whether this is a real problem or not, consider the photographic images below taken of some of our diamonds using the UV light source.
At left we have three basic stages of UV reaction: Strong, Moderate and Faint (to Inert). While the colors imparted by UV on these diamonds appear to be a white color, causing the diamonds to color grade higher, consider the next image below regarding the impact of UV on diamond grading.
Difficult to photograph in UV light but the impact is none-the-less dramatic. The yellowish green color of the UV reaction of this diamond would cause the color grade to be significantly lower than the actual diamond color grade would be otherwise. In this case, the impact of UV on the diamond color grade is unnecessarily negative, and could cause a diamond sold at one color grade to be incorrectly graded far lower by someone not using the UV protected diamond color grading set up we have done here.
Since there are no established industry standards for diamond color grades or color grading procedures, it is difficult to say that a set-up with the above impact should or should not be used based on industry requirements. However, in order to be as accurate as possible, (which should be the goal of everyone in the ISG Global Community), the use of any tool or technique that will guarantee the most accurate grading should always be the preferred internal operating procedure.
Sometimes it's the little things that set the true professionals apart from the rest of the industry. I believe this is one of those little things.
Robert James FGA, GG
President, International School of Gemology
If you have problems viewing the images, visit the ISG Research page to see this article in its entirety: https://schoolofgemology.com/Research/Fluorescence.html
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