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Diamond Fluorescence: The Bad and the Beautiful

Have you ever been out somewhere and noticed a diamond that looks particularly bright and glowing, you can see it dancing from across the room? Or perhaps you’ve seen a diamond that looks dreary and dull, like it needs a good cleaning. Both of those looks could be caused by the same phenomenon – fluorescence.

For decades fluorescence has widely been considered a “bad” characteristic for a diamond, dragging down the value of the gem. But is it really bad, or can it have a positive impact on a diamond?

What is Fluorescence?

Fluorescence testing
Diamonds without fluorescence and with fluorescence, photo courtesy IGI.

Fluorescence is visible light given off by a diamond when it is subjected to ultraviolet (UV) rays, which are part of daylight. A diamond with strong enough fluorescence will have a noticeable  glow when it is hit by UV light. This does not affect the durability or strength of the diamond, however it may affect the value and look of the stone. Among natural diamonds, anywhere from 35 to 50 percent of those examined by a lab will have some level of fluorescence. That ratio is notably less for laboratory grown diamonds.

According to Steven Rees, Executive Director, IGI North America Grading Laboratory:

We see fluorescence in about 10 percent or less of lab grown stones. The majority of those that have any are CVD (Carbon Vapor Deposition) grown, and tend to emit very slight yellow fluorescence. Only about one percent or less of HPHT (High Pressure High Temperature) produced stones emit fluorescence. When present it tends to be slightly orange.

What About Blue?

Diamond exhibits strong fluorescence, photo courtesy IGI.

In natural diamonds fluorescence typically causes a faint bluish light to emit from the diamond. This bluish light can nicely improve the look of a diamond in the H/I/J/K color range. That’s because the blue tone diminishes the yellow hue that start to appear in diamonds in this range of the color scale. However, if a diamond is a D/E/F color and the fluorescence is strong, it may be detrimental to the appearance of the stone, creating a hazy dullness, sometimes described as milky.

Hazy diamond
Photo courtesy of

Rees continues:

At IGI we grade fluorescence as none, very-slight, slight or strong. UV must be present to excite fluorescence, so without a UV source there is no impact. Daylight has UV, but very slight fluorescence is still hard to detect on a sunny day. Slight may possibly be seen. Strong fluorescence is rare but it’s typically observable in daylight.

Diamonds display three levels of fluorescence, very slight, slight, and strong, photo courtesy of IGI.

How Do You Decide if Fluorescence is Good or Bad? 

If you know that a stone has fluorescence,  look at it under different light sources to see how the diamond changes. Fluorescence lasts only as long as the diamond is exposed to UV light, so the stone changes according to the light. The same stone may look different when viewed in different light. The diamond should stay bright and glowing under any light source.

Before deciding that a diamond with fluorescence is not for you, examine it carefully, because it may actually be just the stone for you – a lower color that looks brighter and whiter than its equivalent counterparts at a more attractive price.

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