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What Is Diamond Fluorescence? What Is Fluorescent Grading?

What Is Diamond Fluorescence? What Is Fluorescent Grading?

When people think of diamonds, a few characteristics come to mind first – such as clarity, cut, size (often confused with carat weight), and color. There are some less known – but no less critical – features that should be added to the list. 

You’ve probably heard about luminescence in the animal kingdom – but jewelry can glow, too. Wait, what? Yup, it’s true – diamonds never cease to amaze you, huh? That probably got you wondering: What is diamond fluorescence? 

In this article, we’ve revealed everything there is to know about this phenomenon – and how this affects other attributes of gemstones. Plus, we shared some tricks to using that knowledge, too.

So, without further ado, let’s get started! 

What Is Fluorescence And How Long Does It Last? 

Fluorescence is a natural occurrence that can give some diamonds a special glow. The energy of visible light – in the form of photons – excites atoms of the diamond, and they reemit that light almost immediately – within about 10−8 seconds. 

Unlike phosphorescence, which requires additional excitation and causes a glowing effect that can last for years, fluorescence is much rarer and occurs in 0.01% of stones. 

Since the reemission happens so quickly, the fluorescence comes to an end the second the light source is removed. 

How Does Fluorescence Happen? 

There are hundreds of different luminescent centers that can be found in a diamond – and we’re still counting!

This ample variety of luminescent centers is less common in other minerals, though. It’s a result of the diamond’s unique chemical and physical structure. You can learn about those structures here, by the way.

Diamonds have the highest known atomic density of all solids. Their carbon atomic network is so dense that any foreign atoms – such as those from a light wave – will strongly interact with the lattice electrons. 

Light photons “excite” the diamond electrons, making them change their energy state – and the emission of photons occurs when these atomic particles start “relaxing” and returning to their ground state. 

In the mentioned color centers, there are impurities – trace elements, such as H, He, Ne, Xe, Li, B, N, O, P, Si, Ge, As, Ti, Cr, Ni, Co, Zn, Zr, Ag, W, and Tl – that can be found within gems. And as it turns out, they impact the fluorescence details!

While we’re at it, four factors contribute to this glowing phenomenon: 

  • The large bandgap of diamonds (5.49 eV) is favorable for luminescence; these energy transitions require both ground and excited states to lie within the bandgap. 
  • Diamonds have the highest Debye temperature of any solid (∼2000 K), which means that photon-electron coupling doesn’t pepper the ground-to-excited state of diamond electrons. 
  • Diamonds also possess the most expansive optical transparency window of all known solids – from 0.22 μm to the far-infrared. That means that the UV light can serve as a luminescence source, along with the visible and the IR spectral regions. 

The unmatched luminescence photostability comes from their high mechanical hardness and thermal conductivity. There is an importance here, too – but we’ll get to that later.

What Colors Do Diamonds Fluoresce? 

In general, diamonds fluoresce in many colors – notably blue (about 95%), green and yellow. But sometimes, there can be found those that shine white or bright yellow.

How Can The Glow Be Seen?

Fluorescence is checked by using a UV light, and this method can have one or two benefits. 

Not only will it show the nuance of the glow, but it can also help identify the diamond jewelry – if a photographic record is taken, that is.

Since no two diamonds are alike, neither is their internal structure. Pieces that come from the same stone may appear to be a perfectly matched pair in ordinary light, but they might show very different fluorescent characteristics.

How is that possible? Well, they can’t have an equal number of chemical impurities or lattice radiance defects; that’s not how nature works.

Nowadays, gem colors are graded under a daylight-equivalent light bulb with a UV component. That stimulates natural lighting, meaning the color and the fluorescence can be checked at the same time. 

You can check out our article on diamonds and black lights if you want to know which lamps are used in the process! 

Can Fluorescence Be Faked?

Fluorescence can also indicate if the gem is real or not. In a natural stone, fluorescence occurs equally, whereas synthetically created stones glow in a criss-cross shape or hexagon due to the precious stone being formed quickly.

Related Read: Do Fake Diamonds Glow Under Ultraviolet (UV) Light?

Are All Diamonds Fluorescent? Does This Affect The Value Of The Gem? 

About 30% of natural diamonds are fluorescent, and only about 2 to 10% will have an intensive glow. Their color centers are an area of active research both in the gemological community and the nanotechnology arena. 

Scientists call them Type I diamonds, while those that don’t display fluorescence are called Type II.

There are treatments of natural diamonds developed to increase their transparency and, with it, their price, as well. 

One of those, known as HPHT, is accompanied by reduced fluorescence of colorless diamonds, which is salient since fluorescence minimizes the value of colorless diamonds and can result in discounts ranging up to 15%.

Related Read: What Is The Difference Between HPHT And CVD?

However, there’s something that has bothered jewelers since the initial discovery of fluorescent gems about a century ago. 

Before modern-day scales, they were graded based upon their “true color” – as seen indoors, in artificial light. 

But, in daylight, additional factors such as time of the day, geographic location, and cloudiness cause the color of these diamonds to change. 

So, there was a dilemma: What lighting should be used in lab-based color grading? 

Scientists wanted to strip the lighting used for diamonds of UV waves since it problematized the gem’s value appraisal. For example, gems that appeared yellowish under artificial light seemed to be bluish in daylight. “Northern daylight” was now the standard. 

After doing research, they came to a conclusion: “Indirect daylight through the windows has about as much UV radiation as the fluorescent light sources.”

UV lighting has been used ever since. It shows the actual color of a diamond, as it often seems to be of higher quality when looked at indoors.  

What Is Fluorescent Grading? 

If you read about diamond quality before, you most certainly came across the name GIA – the Gemological Institute of America. Established in 1931, it’s the world’s leading authority on all things diamond-related and offers research, education, and laboratory searches. 

They did the previously mentioned research about UV vs. indoor lighting for diamond grading, too.

Since fluorescence can correlate with color, we should start with their color scale. Now, the rule of thumb is: The lesser the color, the higher the gem’s value. 

It starts with the letter D and goes all the way to Z. The alphabet reflects the shift from highest valued colorless D-grade diamonds to slightly brownish or yellowish Z-graded ones.

That’s a topic that deserves a whole separate discussion, though. If you’re interested, you’ll find a deeper analysis of the famous scale right here.

GIA still holds a reputable fluorescence grading scale they’ve established, used by all jewelers and many gemologists around the globe:

  • None – Not even a soft glow under UV light from the diamond, since there are no trace elements to cause fluorescence; they cost the most
  • Faint – A soft blue diamond glow under the UV light, caused by minor trace elements that have no real impact on the diamond; has an almost identical effect on the price as the “None” graded one
  • Medium – A light blue glow is emitted from the diamond under the UV light; provides excellent value, but reduces the price, and can improve the color of lower grades such as J, K, or L
  • Strong – A deep blue glow is emitted when the diamond’s exposed to UV light; they can slightly improve lover colors such as J, K, or L, but can have a negative effect on high color diamonds such as D or E – which appear slightly grayish 
  • Very Strong – A very bright blue diamond glow under the UV light that could affect the diamond’s appearance; the diamond appears grayish and hazy

The AGS has a different scale that identifies None and Faint together as negligible, so don’t be surprised if you come across this one, as well. 

Is Diamond Fluorescence Good Or Bad? 

The jewelry industry hasn’t given a definitive answer to this question. It mainly depends on the diamond itself, the expert’s subjective opinion, and current trends. 

With that said, there are some instances where fluorescence could affect the diamond’s value.

Suppose that a stone fluoresces so strongly that it appears somewhat hazy, oily, or cloudy. In that case, this phenomenon will decrease the price to a reputable jeweler – since the diamond can lose its capacity to refract light the way it’s supposed to.

Granted, there are other reasons why a diamond can get cloudy, but we won’t go into the details here. We have another guide that covers this topic! 

Some geologists think this effect contributes to the uniqueness of the gem. But if you disagree, don’t worry – significant fluorescence-related haziness is relatively rare. 

In other cases, it can also make a diamond’s body color appear more white and make it seem more expensive. 

For instance, if you decide on an “F” colored diamond that isn’t colorless but still delivers intense fluorescence, that could make it look as if it’s the most expensive clear, colorless gem. 

That only goes for diamonds that shine blue, though. For those that have a yellowish hue when glowing, it’s the opposite. But if the gemstone is yellow-colored, with a blue glow – this makes it appear lighter and, in turn, boosts its price.

With that said, you should generally concentrate on the four C’s – color, clarity, carat, and cut – since the fluorescence is almost always negligible. 

Learn More: 4 Cs Of Diamonds – Order Of Importance: The Most Important C?

Does Diamond Fluorescence Affect Value? 

As previously stated, fluorescence can affect the color – and, in some cases, clarity – and almost always the gem’s price – for better or worse. But here’s what you should remember: A somewhat fluorescent gemstone without the UV light is pretty much the same as the one that doesn’t glow. 

An expert should always inspect the diamond since the price varies depending on the jeweler’s experience in noticing the subtle nuances. You should also check the piece in person since the possible visual effect might not be listed in the gem’s description – if you’re shopping for a gem online, that is.

Diamonds With & Without Fluorescence: Can You Tell The Difference? 

A GIA fluorescence study proved that the strength of the fluorescence of the gem almost always has no visible effect. As it turns out, the average person can’t distinguish which gemstone glows and which doesn’t – unless looking under a UV light, of course. 

So, if you’re worried about the fluorescence affecting the overall appearance of your gem, we can assure you that most people won’t be able to tell the difference. 


What is diamond fluorescence? If you’re still here, you know the scientific explanation behind fluorescence and just what to take note of when at a jewelry store. Here’s a quick reminder of everything we’ve discussed:

  • Always ask for a fluorescence check besides having the certificate; you should know the actual quality and appearance of the gem before buying.
  • Don’t be surprised if all the stones in your jewelry piece don’t show the same UV-induced glow – very few do!
  • Choose a lower color quality gem that shows more fluorescence to stay on budget; you’ll get the same effect. The usual blue glow has a better discoloring effect on the diamond’s body than any other.

And remember that fluorescence is primarily an identifying characteristic of a gem – it’s not good or bad. The best way to put it would be – “It depends.” Now that you understand its impact, you can make a more informed purchase decision, that’s for sure.

Related Read: What Are The Characteristics Of A Diamond?