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Diamond Fluoresence

What is Diamond Fluorescence?

Diamond Fluorescence is the glowing colour that appears when certain diamonds are exposed to long-wave ultraviolet (UV) light. As soon as the ultraviolet light source is removed, the fluorescence ceases. It is a naturally-occurring optical phenomenon and is completely harmless!

What are the various grades of Fluorescence

None (GIA terminology) or Inert (AGS terminology): no fluorescence at all
Faint (GIA) or Negligible (AGS): a very slight glow of colour that is difficult to see under ultraviolet light
Medium (GIA & AGS): a light glow of colour under ultraviolet light
Strong (GIA & AGS): a deep, even glow that is evident under ultraviolet light
Very Strong (GIA & AGS): a very deep, even glow that is very evident under ultraviolet light

Why is Diamond Fluorescence good for lower colour grades?

The bluish colour of Very Strong fluorescence can actually cancel out the yellow body colour in diamonds with a colour grade of I or lower, giving them a more colourless face-up appearance in sunlight (which is a natural source of ultraviolet light).

This means that Very Strong fluorescence allows a customer to purchase a lower-coloured diamond that will appear more colourless than it actually is, for less money than the customer would pay for a higher-coloured diamond.

Is Diamond Fluorescence bad for Diamonds with High Colour grades?

Actually, in most cases, fluorescence is not bad at all. This is a myth that was spread for years in the jewellery industry, but that has recently been disproved by the Gemological Institute of America (GIA).

While a very small percentage of diamonds with Very Strong Blue fluorescence display a haziness or cloudiness that is referred to as “overblue”, this condition is so rare that most jewelers will never even encounter an overblue within their lifetime. At Daniel Prince we never sell overblues.

In 1997, a panel of experts at the GIA completed an extensive study involving more than 1,000 diamonds and discovered that, for non-trade observers (that is, the majority of the jewellery-buying public) the difference between varying levels of fluorescence was indistinguishable. Specifically, for most consumers, fluorescence had no visible effect on a diamond’s colour appearance or transparency.

GIA’s conclusion: Fluorescent diamonds are stigmatized needlessly, which puts a negative impact on their salability.

Interesting Facts about Diamond Fluorescence

To understand how little difference there is between non-fluorescent diamonds and strongly-fluorescent diamonds, compare the diamonds in these photographs:

The photo of the left shows the necklace and earrings under normal lighting conditions. In this normal daylight, the diamonds all appear to be perfectly matched in colour; it is impossible to guess which diamonds have no Diamond fluorescence and which diamonds have medium or even very strong fluorescence.

The photo on the right shows the same necklace and earring set under an ultraviolet light.

Harry Winston Necklace

Credits: Photos from Gems & Gemology, Winter 1997, Volume 33, Issue 4. Photos by Harold & Erica Van Pelt. Jewellery courtesy of Harry Winston, Inc.


Fluorescence in diamonds is considered rare. 65% of diamonds exhibit no fluorescence at all.

“Overblues” are so rare that when GIA conducted an extensive study on the affect of fluorescence in diamonds, no overblues were found among the 1,000+ diamonds that were used for the study.

Fun factoid: The famous Hope Diamond displays an extremely rare and unusual red fluorescence, rather than the normal blue fluorescence, and this has influenced the legend of its curse!