Apart from the four Cs(*1), one of the most frequently cited properties of a diamond is fluorescence. Diamond fluorescence, by definition, is the degree to which it gives off a colored hue when exposed to ultraviolet radiation. In some circles, fluorescence is considered an undesirable trait for a diamond. So, some vendors and buyers have lower expectations for the prices of diamonds(*2) that exhibit higher levels of fluorescence.
However, this is not a universally accepted position. There is an ongoing, heated debate regarding what impact a diamond’s fluorescence has on its appearance to the naked eye under normal conditions. There is no debate as to whether fluorescence actually exists or not, as a diamond’s fluorescence or lack thereof is clearly visible when the diamond is placed under a black light. Obviously, the people in the “fluorescence is bad” camp are of the opinion that fluorescence has a clear and detrimental effect upon the appearance of a diamond even when viewed in normal light.
Then there are those who take a more nuanced view of fluorescence. Some argue that, although any fluorescence in diamonds with good color ratings is a negative, a small to medium amount of fluorescence can actually improve the appearance of diamonds with lower color grades. Some also believe that fluorescence can improve the overall appearance of naturally colored diamonds.
To some, fluorescence itself is an impurity that afflicts some diamonds. In these cases, as with those who simply believe that fluorescence negatively affects a diamond’s appearance, this naturally leads to a reduction in the value of diamonds that exhibit fluorescence, and the more fluorescent a diamond is, the more its value suffers in these circles. One prominent voice on the anti-fluorescence side is the respected Rapaport Report, a diamond industry publication that is often cited as the standard in diamond valuation.
On the other hand, there is another side that posits that fluorescence has absolutely no meaningful impact on the quality or appearance of a diamond, and therefore that fluorescence should not factor into the pricing of a diamond. The basic foundation of this position is the argument that it isn’t really possible to distinguish diamonds of differing fluorescence from one another with the naked eye under normal light conditions.
This position appears to be backed up by a greater body of evidence. Most significant are the results from a study carried out in 1997 by the Geological Institute of America, which found that even individuals involved directly in the diamond trade, including professional diamond appraisers, were unable to consistently identify or discern differing levels of fluorescence in diamonds.
Even more importantly, ordinary individuals involved in the study were unable to discern any effect from fluorescence. This really should settle the debate, as it is the consumer’s tastes that drive the diamond industry, and it wouldn’t make much sense to charge a consumer more money for a particular diamond based on a variable that has no meaningful effect.
Nevertheless, there are still some vendors who will downgrade the price of diamonds that exhibit higher levels of fluorescence. For consumers, this can represent a good opportunity. If you’re looking at two diamonds with the same ratings in every category except for fluorescence, and the vendor is selling the more fluorescent diamond at a lower price, you essentially have the chance to buy the “same” diamond for less.
*1 – “Diamond Ring Grading” article
*2 – “Diamond Prices” article