Fluorescence… Does your diamond have a secret?

By Megan Reynolds, AJP (GIA)

Fluorescence is defined as “the emission of visible light by a diamond when it is stimulated by invisible ultraviolet rays”, or in English, when a diamond glows under a UV light like a black light. The glow can be many colors from blue, to green, yellow, orange, even red, and lasts as long as the diamond is exposed. Over 95% of diamonds that are fluorescent show the blue color. Let’s play a quick game. Can you tell which of the diamonds in this set are fluorescent? I’ll let you know the answer towards the end of this post.

Photo: Harold and Erica Van Pelt

The GIA regards fluorescence as an identifying characteristic and categorizes it into five groups from diamonds that show no fluorescence, to those that show faint, medium, strong and very strong fluorescence. Of all diamonds, only 25-35% shows any fluorescence at all, and only 10% of those fall into the medium to very strong groupings.

The relative popularity of fluorescence has fluctuated over the years. In the early 1900’s, diamonds in the D to F color range with strong fluorescence were called ‘blue white’ and demanded a 10% increase in price. This sales strategy unfortunately, was the beginning of the end for fluorescent popularity. In efforts to raise profits, less than scrupulous jewelers attempted to attach the term and it’s matching premium to lower colored gems as well and ultimately the FTC was forced to step in and ban the use of the term ‘blue white’.  

In the 1970’s these glowing gems took another hit when enterprising jewelers started selling what they called ‘milky D’s’ at steep discounts. What they were referring to, were D color diamonds with very strong fluorescence. The ‘milky’ part of the name comes from the unfortunate effect found in an extremely small percentage, less than .02%, of diamonds with very strong fluorescence. In these diamonds a hazy or oily appearance is noted, called ‘over-blue’, which is only present in light sources containing UV light. However, the marketing strategy did nothing but hurt the reputation of fluorescent diamonds. Shoppers assumed that the effect must be present in lower color stones as well and the negative impact eventually spread down to stones with weaker fluorescence.

The stigma against fluorescent diamonds continues to this day and in order to combat the negative stereotypes, the GIA performed a study of thousands of diamonds. They placed them into groupings by color and clarity and had both average consumers and experts examine them for fluorescence in both the table up and table down position. It was found that for the overwhelming majority of diamonds, the strength of fluorescence has no widely noticeable effect on appearance or transparency when viewed by the general public. For experienced observers, fluorescence only had a discernable effect in the table up appearance, and it was overwhelmingly positive. The effects were most pronounced in the lower color grades including colors I and K. The reason being that blue fluorescence in lower colored stones can create a ‘false white’, which is a stone that looks much whiter than it is due to the blue fluorescence and the faint yellow body color cancelling each other out. Even near colorless diamonds can have their face up appearance improved by blue fluorescence. In fact, many consumers tended to prefer the look of diamonds with medium or strong blue fluorescence.

Unfortunately these findings haven’t had much effect on the buying strategies of the general public. And it’s almost entirely due to fear of the ‘over-blue’ diamond. However, when the GIA looked to expand its study to include these ‘over-blue’ diamonds, they could not find enough to make a sample. They are exceedingly rare, so much so, that many diamond enthusiasts seek them out for their collections.

The truth is, in real life, it is almost impossible to tell whether or not a diamond is fluorescent just by looking at it. For example, I have a three stone engagement ring. All of the stones match perfectly with regards to color and clarity, but writing this post made me curious. So I went into our supply closet with a black light and turned the lights off. One of my side stones immediately glowed bright blue! And the funny thing is, even though I know that one of them is fluorescent, I could not tell you right now, which one it is. They look exactly the same. In fact, diamonds in a range of fluorescent strengths and colors are routinely placed next to inert diamonds, and yet the piece maintains a uniform overall appearance under normal lighting. Remember the necklace and earrings from the beginning of this post? Here’s the same set under black light. How did you do?

Photo: Harold and Erica Van Pelt