Understanding Diamond Fluorescence
You have probably noticed that some dealers make a big issue of the blue fluorescence that certain diamonds exhibit under ultraviolet light.
In the past, the 4 Cs — carat, cut, clarity and colour — were used to describe the rarity of a polished diamond and to determine its value. However, this was never intended to be the whole story, as diamond experts traditionally preferred to value a diamond on its size, beauty and sparkle.
When dealer price lists became available to the public and buyers increasingly wanted to purchase at or below list price, profit margins narrowed significantly. Dealers therefore began looking for reasons why two visually similar diamonds should differ in price.
The fact that some diamonds glow blue or yellowish under ultraviolet light while others do not provided such a reason. Today, a white diamond with strong blue fluorescence is often priced 10–25% below a similar diamond that shows no fluorescence. The explanation commonly offered is that the blue glow may cause a diamond to appear whiter than its true body colour, resulting in a colour grade on the grading report that appears too high and therefore justifies a discount.
The Facts About Diamond Fluorescence
- At the beginning of the 20th century, colourless diamonds with blue fluorescence were among the most prized. They were known as “blue-white” diamonds and commanded significant premiums.
- In 1997, Dr Tom Moses, then Chief Research Scientist at GIA, published the landmark study A Contribution to Understanding the Effect of Blue Fluorescence on the Appearance of Diamonds. The study demonstrated that fluorescence effects are governed by crystal transparency, not fluorescence strength or body colour alone.
- Gem-quality diamonds with high transparency tend to appear brighter and whiter under the influence of fluorescence, while diamonds with inherently poor transparency may appear hazier.
- The study challenged the long-held trade perception that fluorescence usually has a negative effect on better-colour diamonds and concluded that each diamond should be assessed on its own visual merits.
- GIA found that in approximately 80% of cases, blue fluorescence improves the overall appearance of a diamond.
- Claims that blue fluorescence always causes a hazy or oily appearance — and therefore requires automatic price deductions — are incorrect.
Professional Advice
A diamond should always be viewed in person rather than judged solely from a grading report or a photograph. In most cases, blue fluorescence improves the appearance of a diamond, but in certain instances it can reduce transparency and justify a price adjustment.
Professional evaluation under controlled lighting conditions is therefore essential.
If visiting in person is not possible, the experience and integrity of our gemmologists will guide and advise you appropriately.
Important Note on Diamond Identification
Please note: Fluorescence alone is not a reliable indicator of whether a diamond is natural or laboratory-grown. Both natural and laboratory-grown diamonds may fluoresce under ultraviolet light. Accurate identification requires professional gemmological testing.
By Dr Petre Prins
Managing Director, Prins & Prins Diamonds
PhD, MBA | Geology, Mineralogy & Geochemistry
Cape Town, South Africa