"Foxfire" diamond in the rough, origin Diavik Mines; Approx. 187 carats. Photo by Michelle Z. Donahue, Courtesy Smithsonian Institute

Fluorescence in Diamonds...Good, Bad or Ugly?

Demystifying the misinformation surrounding fluorescence in diamonds


What is Fluorescence?

Fluorescence is the glow sometimes seen when an object emits visible light. Some diamonds fluoresce when they are exposed to ultraviolet light. This can cause them to emit a bluish glow of varying intensities and less often so - a yellow, green or even a white glow.

Most diamonds found in nature do not fluoresce. It has been established over the years, through extensive testing that approximately 30% to 35% of all natural diamonds exhibit some degree of fluorescence when exposed to UV light.

A diamond might fluoresce under a bright sun, at the dance floor or in other places where strong fluorescent or black light is present. Fluorescence is a temporary phenomenon and once the UV light source is turned off, the diamond stops fluorescing.

Diamond grading reports in general describe the intensity in 5 grades of fluorescence as: None, Faint, Medium, Strong and Very Strong. If the fluorescence level is Medium, Strong or Very Strong, the 'colour type' of the fluorescence is also stated on these reports

If you’re still wondering what diamond fluorescence is, think about how ultra-violet light makes your whites look whiter and some of your teeth glow at the discotheque. In the same context, you are reading this because you want to know when a diamond fluoresces or radiates a glow, is that good or bad or neutral for your diamond.

Here is an attempt to demystify some of the misinformation and conjecture surrounding fluorescent diamonds, so read on . . . 

Causes & Effects of Fluorescence

Fluorescence in diamonds remains a widely misunderstood concept, both at the trade and consumer level. It’s been a frequently debated topic within the diamond industry and the opinion of experienced jewellers on this subject tend to differ.

When it takes over a billion years to form in the earth’s mantle, you will likely absorb atomic quantities of your neighbouring minerals and gases. Trace elements absorbed in these minuscule quantities during the growth phase, such as aluminium, boron or nitrogen can cause diamonds to fluoresce. The variations caused in the atomic structure of the diamond crystal act as a trigger.

Ultraviolet light, which consists of high-energy waves, then does the trick, causing the electrons of these trace elements to pop into higher energy states. Electrons in these excited states continue to release that stored energy as light so long as the UV light is present. Remove the UV light, and the excited electrons gradually return to their original state.

Fluorescence in a variety of colours: Photo courtesy GIA | Facets SingaporePhoto courtesy GIA.
Here is a example of very strong fluorescence in a variety of colours. Notice the extreme haziness of the two yellow diamonds exhibiting green fluorescence.
Diamonds can fluoresce in a variety of colours - primarily blue but the visible colour range includes yellow, orange, white, green and very rarely red. Blue is by far the most common colour of diamond fluorescence occurring in over 95% of fluorescent diamonds.

When a diamond exhibits fluorescence, the perception of its face-up colour is affected in unpredictable ways. I use the word ‘perception’ emphatically because that is precisely what it is. It’s the perception of colour that changes and never the true colour itself.

Fluorescence can be good or bad for a diamond’s perceived colour; faint to medium can improve a diamond’s colour and strong to very strong can oftentimes make the diamond look milky or hazy. Many trade professionals think that moderate levels of blue fluorescence can enhance a diamond’s appearance whereby it shows up one colour higher than its actual colour. Sometimes even two colours.

In the near colourless range (I to M colours) faint or medium blue fluorescence can negate the effects of a diamond’s colour tinge. It can make a diamond with a yellow tinge appear less yellow (read: whiter) in the presence of natural daylight. We know that natural daylight always has a UV component – less or more depending on geography and ambient conditions.

About a third of all natural diamonds display some level of visible fluorescence, and less than 5% of these exhibit fluorescence in the strong to very strong category which could alter their appearance to a degree - even under regular viewing conditions, i.e. non-UV light.

Very strong blue diamond viewed in daylight | Facets SingaporeA very strong blue fluorescence in a diamond may show up as a milky haze even under normal daylight conditions.                                  
                                      Photo courtesy: diamondreview.com

Under rare circumstances, this strong to very strong fluorescence can cause the diamond to appear oily or hazy under normal daylight. This haziness can interfere with the transparency of the diamond and in turn the light-return-factor which in turn directly impacts its brilliance. Since the most valuable and desired feature of a diamond is its brilliance, anything that diminishes or takes away from the diamond's sparkle should be avoided if the intended use is for a piece of jewellery. Except if it is intended as a collector’s item, then in a quirky way these ugly diamonds are rare. Ever heard of a rare ugly duckling?

“In some cases the stone can look ugly and should be avoided except as a collector’s item because in a quirky way, these ugly diamonds are rare"

Faint 'yellow fluorescence' can have a markedly negative effect on a diamond’s colour. Say you have an I, J or K colour diamond with yellow fluorescence. In full sunlight, the yellow fluorescence is sure to intensify the faint yellow colour . In addition, diamonds with yellow or white fluorescence tend to have hazy or milky hues.

Determining and Measuring Fluorescence

GIA, the world’s premier grading laboratory and education institute for all things jewellery considers diamond fluorescence an identifying characteristic. It is not a factor considered in grading of the universal 4Cs but is an important element of the overall grading report that enables a diamond to be valued. All laboratory grading reports describe a diamond’s fluorescence in five levels of intensity as described earlier.

Increasing levels of diamond fluorescence | Facets SingaporePhoto courtesy: withclarity.com

GIA conducted a study some twenty years ago to understand the effect of blue fluorescence on diamond appearance. The study investigated 4 sets of diamonds, in four different colour grades (E, G, I, and K) but with differing intensities of blue fluorescence. Otherwise the diamonds in each set were as similar as possible in all other respects (size, cut grade, clarity etc). Diamond graders, trained professionals, and average observers viewed the diamonds under controlled conditions to make a judgment about their appearance.

See also: Our guide to Insights into the Diamond Industry

Here is what the study concluded: “For the average observer, representative of the jewellery buying consumer, no consistent effects of blue fluorescence on the face-up appearance of the groups of diamonds were observed. Even experienced observers did not consistently agree on the effects of fluorescence from one stone to the next”. The operative word here is ‘consistent’. Ever since, fluorescence has been one of the most hotly debated topics in the diamond industry.

Price Impact of Fluorescence on Diamonds

Opinions on the effects of fluorescence range across the spectrum. See if you notice any visible effects of fluorescence the next time you are shopping for a diamond in a jewellery store – and then decide if fluorescence appeals to you. And I hope this article will simplify your decision making, if you do decide to take home a fluorescent diamond.

In assigning a diamond’s colour grade, labs must examine the gem in a controlled lighting environment, designed to minimize the influence of external fluorescence. This is done in order to arrive at an accurate and objective assessment of the diamond’s colour. The UV component in natural daylight can cause a shift in colour perception if the diamond is inherently fluorescent.

On the one hand, we could perceive fluorescence as resulting in an improvement in the diamond's colour grade thus increasing its desirability (and worth) in our minds. As a result it would appear more valuable and encourages us to pay more for it. That is how the world turns.

"Fluorescence can enhance a diamond’s colour under certain types of lighting and viewing conditions.”

And the opposite certainly holds true. A hint of a lower colour or a perceived influence of colour on an already good colour results in a lower price expectation. 

Here are examples of when fluorescence can lower a diamond’s price:

  • Strong or very strong blue fluorescence : Diamonds in all colour grades could appear hazy or cloudy but this is not a generalisation; each diamond is unique and the reaction to strong fluorescence is just as unpredictable. The price impact here is the greatest.
  • Medium blue fluorescence : In the case of colourless grades (D to G) – in certain cases the medium blue intensity can mask the true colour of the diamond and result in a lower colour perception. You pay for an F, and what you see is a G. That calls for a discounted price!
  • Faint blue fluorescence : For colourless grades (D to G), it can have from a positive to no impact on perceived colour but a real influence on the sentiment of the buyer – a common sentiment expressed is if  “I am paying good money for a high colour I don’t want fluorescence interfering with the colour of my diamond even if I don’t see it”. It is a fair reason for hesitation and works to the consumer’s favour. Market forces dictate that the price should drop to a level such that the consumer is willing to compromise. Such a diamond usually sells easily when the price discount makes it worthwhile.



The impact of fluorescence on diamonds and its effect on value is not a simple question, and there isn’t a simple answer. Jewellery professionals disagree about when and how much fluorescence detracts from the value of a diamond. My conclusion is it has three effects- in some cases a good effect, in more cases a bad effect and in some less frequent cases an ugly effect.

Any degree of fluorescence in diamonds in the colourless range (D to G) is considered less desirable going by market perception, hence their price and values are also lower. In my opinion though, fluorescence per se isn’t always a negative trait, which means you can save money by choosing a diamond with some level of fluorescence if it is combined with the advice from a jeweller you trust.

"There are no defined rules to help you along the way, so let your head guide you but let your heart lead the way"

Diamonds in the near-colourless grades (I to M) can benefit from a faint or medium blue fluorescence. In normal daylight viewing conditions, this can make the diamond appear one whole colour grade higher. It follows that you can get good value by choosing a diamond with a lower colour that has been enhanced by its fluorescence. Again, there is a need to seek the advice of a trusted jeweller during an in-person visit. Physically viewing this diamond helps you make an educated choice. Online stores cannot provide this valuable service.

Percieved colour enhancement in J colour due to fluorescence | Facets Singapore'J ' colour stones viewed in standard daylight conditions. Notice the visibly whiter colour tinge of the stone on the right.                         
Photo courtesy: withclarity.com

Twenty or more years ago, fluorescent diamonds in lower colour grades (I-M) attracted decent premiums over non-fluorescent diamonds in Asian and US markets. Because these diamonds looked whiter than their non-fluorescent counterparts and the manner in which fluorescence enhanced their face-up colour made them unique! However over the past 15-18 years, things have changed with the internet hastening the spread of incomplete knowledge, thus forcing the market to discount fluorescence in all colours grades. Today the rule of thumb is “higher the colour, the more fluorescence negatively affects price”.

We at Facets have consistently recommended that a diamond should be physically inspected,  whenever possible, before a buying decision is made. This holds even more water if one is shopping for a fluorescent diamond which should be viewed in a variety of lighting conditions. You will be surprised by what you see and don’t see: Unfortunately, for online shoppers - even with today’s advanced photography it is not possible to accurately visualise the face-up look of a fluorescent diamond in natural light.

I hope by now you have some clarity on the complexities of fluorescence in diamonds and are able to appreciate the role of a jeweller at the street level. Online bargains are plenty to be had, but don’t let price and slick marketing be the main influencing factor in your buying decision. The jewellery professional can help you make the right decision and save you money in the long run.

I leave you to draw your own final conclusions, but one thing should by now be clear - there is no one right or wrong answer to the many mysteries of fluorescence. And fluorescence can have all three effects on a diamond - good, bad and ugly.
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Suresh H.

Written by Suresh H.

Respected international diamantaire and industry expert with a deep knowledge of the industry, GIA trained diamond grader, researcher and writer about all things related to diamonds.

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