What Is Fluorescence In A Diamond?
Fluorescence in a diamond under UV light
Fluorescence is one of the most misunderstood aspects of a diamond. You may have noticed the term “fluorescence” on a diamond certificate before making a purchase and wondered — is it a good or bad thing?
In simple terms, fluorescence refers to the soft blue glow that a diamond emits when exposed to ultraviolet (UV) light. Since sunlight contains UV rays, most of us see this effect without even realizing it. Fluorescent light bulbs also emit UV, so this phenomenon is more common than you might expect.
The blue glow is caused by trace amounts of nitrogen present in the diamond’s atomic structure. Approximately one-third of gem-grade diamonds display some level of fluorescence — a completely natural characteristic.
Two Key Factors to Consider: Colour and Strength
The degree of fluorescence in a diamond, from None to Very Strong
The Fluorescence Colour
About 95% of fluorescent diamonds glow blue under UV light, though rare cases can exhibit yellow or other colours. Blue fluorescence often helps a diamond appear whiter, especially if it has a lower colour grade — effectively improving its visual appeal. In contrast, yellow fluorescence can dull the appearance of a diamond by emphasizing any yellow undertones.
The Fluorescence Strength
Fluorescence is graded from None, Faint, Medium, Strong, to Very Strong. According to the Gemological Institute of America (GIA), fluorescence strength has little to no effect on most diamonds’ appearance under normal lighting. However, in rare cases, strong fluorescence can cause a diamond to appear cloudy or hazy.
Buying a Diamond? Questions to Ask About Fluorescence
A diamond’s fluorescence is noted on a GIA Grading Report
Is Fluorescence Good or Bad?
It depends on the diamond’s colour and intensity. In colourless diamonds (D–F), strong fluorescence might cause an oily or cloudy appearance in sunlight. In near-colourless diamonds (I–J), blue fluorescence can improve the diamond’s apparent whiteness, enhancing its beauty and value.
Does Fluorescence Affect Value?
Yes — but not always negatively. Diamonds with yellow fluorescence may sell at a discount, while diamonds with blue fluorescence that improves their appearance could command a higher price. Each case is different, which is why reviewing the diamond in person is always recommended.
How to Check If Your Diamond Has Fluorescence
You can confirm by viewing the diamond’s grading certificate. GIA-certified diamonds will clearly state if fluorescence is present, along with its intensity: None, Faint, Medium, Strong, or Very Strong. If it’s marked “None,” the diamond does not fluoresce at all.
Final Thoughts
Fluorescence should not be the primary factor in choosing a diamond — focus instead on cut, clarity, colour, and how the diamond looks to your eye. Some buyers prefer the glow and value of fluorescent diamonds, while others prefer stones without it. Ultimately, it comes down to personal preference.
To learn more about the science and beauty behind diamonds, visit the Cape Town Diamond Museum at the Clock Tower, V&A Waterfront. Book your guided tour here and uncover the secrets behind the world’s most brilliant gemstone.