You notice something strange—one of your diamonds glows under UV light, but the others don’t. It’s not something you usually think about, but now you’re wondering why. Is it a flaw? A sign of a fake? Or just a natural quirk?

Diamonds come from deep within the Earth, formed under extreme pressure over millions of years. Along the way, some pick up tiny traces of other elements that change how they react to light. Under UV rays, these elements can make a diamond glow in different colors, most often blue. But not all diamonds have this effect, and the reasons for that are more interesting than you might expect.

What Is Diamond Fluorescence?

Fluorescence is the glow some diamonds emit when exposed to ultraviolet (UV) light. This happens because certain trace elements in the stone absorb UV radiation and release visible light. The most common fluorescence color is blue, but some diamonds can glow yellow, green, white, or even red.

Fluorescence isn’t unique to diamonds—many minerals and even everyday objects, like banknotes or laundry detergent, fluoresce under UV light. In diamonds, this effect comes from their formation process deep in the Earth, where they interact with elements like nitrogen or boron. These impurities don’t always affect the way a diamond looks in normal light, but under UV, they reveal hidden characteristics.

Not all diamonds fluoresce. Some remain completely dark under UV light because they lack the right trace elements or structural conditions for fluorescence to occur. This difference has led to myths about fluorescence being a sign of a fake or low-quality stone, but the reality is more complex.

Causes of Fluorescence in Diamonds

Fluorescence in diamonds comes from trace elements and structural irregularities in the crystal lattice. The most common cause is nitrogen, which can create a blue glow when the diamond is exposed to UV light. In rarer cases, boron or other elements can cause different fluorescence colors, such as yellow, green, or even red.

Another factor is how the diamond’s atomic structure developed over millions of years. During formation, some diamonds experience shifts or distortions in their crystal lattice, which can influence how they interact with UV light. These microscopic changes affect whether a diamond fluoresces and how strongly it does.

The absence of fluorescence doesn’t mean a diamond is more or less “real” or valuable—it simply means the conditions during its formation didn’t introduce the necessary elements or defects. That’s why some diamonds glow under UV light while others remain dark, even if they look identical in normal lighting.

How Fluorescence Affects a Diamond’s Appearance

In most cases, fluorescence is invisible under normal lighting. It only appears when the diamond is exposed to ultraviolet (UV) light, such as a blacklight. However, in certain cases, fluorescence can influence how a diamond looks in natural daylight, which contains some UV rays.

For diamonds with strong or very strong fluorescence, this effect can sometimes make the stone appear slightly hazy or milky, especially in high-color grades (D-F). On the other hand, in lower-color diamonds (G-K), blue fluorescence can actually improve the appearance by counteracting yellowish tones and making the diamond look whiter.

Different fluorescence colors also affect perception. Blue fluorescence is the most common and generally has little impact on value. However, rare colors like green, yellow, or red fluorescence can be considered unusual and sometimes desirable to collectors.

Despite these potential effects, most diamonds with fluorescence look completely normal in everyday conditions, and many people never even notice it unless they specifically check under UV light.

Does Fluorescence Impact a Diamond’s Value?

Fluorescence can influence a diamond’s price, but its effect isn’t always straightforward. In some cases, it can lower the value, while in others, it can enhance the stone’s appeal.

For high-color diamonds (D-F), strong fluorescence is often seen as a drawback because it can sometimes create a hazy or milky appearance, reducing the stone’s transparency. As a result, these diamonds may sell for less than similar stones without fluorescence.

However, in lower-color diamonds (G-K), blue fluorescence can be beneficial. Since blue is the complementary color to yellow, it can make a slightly tinted diamond appear whiter, improving its overall look. Because of this, some buyers seek out fluorescent diamonds as a way to get a better-looking stone at a lower price.

Fluorescence alone doesn’t determine a diamond’s value. Factors like cut, clarity, and overall appearance matter more. Some people prefer fluorescent diamonds, while others avoid them. Ultimately, whether fluorescence is a good or bad thing depends on personal preference and how it affects the specific stone.

Testing for Fluorescence

Fluorescence in diamonds is tested using ultraviolet (UV) light, usually a long-wave UV lamp. Jewelers and gemologists often perform this test to check for the presence, strength, and color of fluorescence. The Gemological Institute of America (GIA) grades fluorescence on a scale from None to Very Strong, based on how intensely the diamond glows under UV light.

If you want to check fluorescence yourself, you can use a blacklight. In a dark room, place the diamond under the light and observe any glowing effect. If the stone emits a blue or other-colored glow, it has fluorescence. However, not all fluorescence is visible to the naked eye—some stones may show a weak reaction only under professional lab conditions.

It’s important to remember that fluorescence alone doesn’t confirm whether a diamond is real or fake. Some synthetic and imitation stones also fluoresce, while some natural diamonds do not. To accurately determine authenticity, fluorescence testing should be combined with other gemological methods, such as checking refractive index, examining inclusions, and verifying certification from reputable labs.

Final Thoughts: Does Fluorescence Matter?

Fluorescence is just one of many characteristics that make each diamond unique. While some stones glow under UV light due to trace elements in their structure, others remain dark. This effect doesn’t automatically make a diamond better or worse—it all depends on how it interacts with the stone’s overall appearance.

For some buyers, fluorescence is a non-factor. Others see it as a benefit, especially when it enhances a diamond’s color. On the other hand, strong fluorescence can sometimes lead to a hazy look in high-color diamonds. The key is to evaluate the diamond in different lighting conditions and decide if it works for you.

At the end of the day, a diamond’s beauty isn’t defined by fluorescence alone. What matters most is how it looks to the eye and how much you love it.