Natural diamonds have a property known as fluorescence, which allows them to produce a brilliance of varying colors when exposed to black light (also known as ultraviolet light). About 25% to 35% of diamonds exhibit any degree of fluorescence in reaction to long-wave UV light, and over 95% of these diamonds have a fluorescent blue color. Rarely do they fluoresce to other colors, such as yellow or green. Fluorescence is not a grading factor like GIA 4C (color, clarity, cut and carat weight), but it is an identifying characteristic.
GIA diamond grading reports and diamond records describe the fluorescence of a diamond by its intensity under long-wave UV light (none, weak, medium, strong and very strong). If the fluorescence is Medium, Strong or Very Strong, the color of the fluorescence will be noted. Some trade professionals think that blue fluorescence improves the appearance of a diamond, especially in diamonds with color grades I to M. Bluish fluorescence can cause a faint yellowish diamond to appear more colorless under UV light, which is part of natural light.
As a result, diamonds with color grades I to N with very strong to medium bluish fluorescence may have a slightly higher price per carat than diamonds with similar color grades that do not fluoresce. The opposite is true for diamonds with higher color grades. In commerce, diamonds in the D to H color range with bluish fluorescence are often considered less desirable than diamonds of similar grade without fluorescence, because some people believe that bluish fluorescence can cause diamonds to have a hazy or oily appearance. In a recent study, the GIA found that blue fluorescence has little or no impact on transparency, except in extremely rare cases where a diamond (such as the Portuguese diamond) has a light scattering defect.
These defects cause haze that can sometimes be intensified with strong fluorescence, thus reducing contrast in the diamond's face-up pattern. However, fluorescence does not cause haze by itself. In addition, this “overblue” hazy effect occurs in less than 0.2% of fluorescent diamonds subjected to GIA. But the association between fluorescence and haze persists, and diamonds in the D to H range with very strong fluorescence often sell for less than diamonds that do not fluoresce, although their color and transparency will most likely not be affected by their fluorescence.
These diamonds can turn out to be a good deal. If you're not familiar with the GIA color scale, learn more with the GIA diamond color chart. No, fluorescence does not cause opacity in diamonds. Fluorescence can increase the pre-existing haze of a diamond caused by light scattering defects, thus reducing contrast in its face-up pattern, but it does not cause haze in and of itself.
Because light scattering defects are incredibly rare, most consumers don't have to worry about fluorescence affecting the appearance of a diamond in most lighting situations, even if the diamond has a strong to very strong fluorescence. But this does mean that consumers should look at a diamond in person. Any fluorescence-intensified opacity, if present, will be noticed (for example) in daylight-equivalent illumination. To study the effect of blue fluorescence on the appearance of diamonds, GIA scientists assembled sets of diamonds of color grade E, G, I and K.
The diamonds in each set were as similar as possible, except for the intensity of their blue fluorescence. Diamond appraisers, trained professionals and average observers viewed diamonds under controlled conditions to make a judgment on their appearance. It seems that for the average observer - intended to represent the jewelry-buying public - no systematic effects of fluorescence were detected. Overall viewers perceived that the deep blue fluorescent diamonds had a better color appearance when viewed at the table.
Most observers saw no relationship between fluorescence and transparency. Diamond fluorescence is neither good nor bad; some people find fluorescent diamonds to be beautiful and fascinating while others don't. Now that you know more about diamond fluorescence you can decide if it enhances or detracts from its beauty when you buy your next diamond.