Diamond Fluorescence - Good or Bad?
The answer is both… or neither.
What is fluorescence in a diamond? When exposed to invisible ultraviolet (UV) light, many diamonds temporarily luminesce, emitting visible light of different colors and different strengths. Sunlight and indoor fluorescent lights contain varying amounts of UV radiation. A significant percentage of diamonds in the market have some degree of fluorescence. The vast majority of those are blue, but in rare cases white, yellow, orange and red fluorescence are also seen.
Diamond fluorescence is caused by submicroscopic structures within the crystal. (Blue fluorescence is most commonly associated with the presence of nitrogen atoms aligned in specific arrays within the carbon lattice.) In most cases fluorescence is simply an identifying characteristic and not a performance characteristic, and is therefore NEITHER good nor bad.
In some cases, strong or very strong fluorescence can make a diamond appear cloudy, diminishing its transparency and eye appeal. Such diamonds are often described as having an oily, hazy or milky appearance. In these cases it is obviously BAD.
Medium or stronger blue fluorescence tends to mask yellow body color so diamonds in the lower color range can appear a shade whiter in lighting environments with sufficient UV wavelengths. In these cases it is 'theoretically' GOOD.
However, recent science has demonstrated that the fluorescent effect is diminished dramatically as the distance from the light source is increased. In almost all normal indoor viewing circumstances the distance from the light source is too great to excite the fluorescence effect. Therefore, the idea that the appearance of diamonds in lower colors is improved by fluorescence is largely unfounded. Therefore, in reality it is NEITHER good nor bad.
A related concern that has been a topic of much discussion in the trade in recent years has to do with the possibility of overgrading of color of fluorescent diamonds. Because laboratory color grading is done in light environments that do contain UV and the grading is done with the diamond in very close proximity to the UV source (typically only several inches away), there is concern that the fluorescent effect can mask the true body color leading to higher color grades for these stones. *Independent surveys have been done that show serious problems in color grading accuracy of fluorescent diamonds (see refenced article below). If the grade on the report is higher than the true body color and the appearance of the diamond does not benefit from the fluorescence under normal observation because of distance from the light source, then fluorescent diamonds actually look worse than the grade would indicate. In this case it is obviously BAD.
Diamonds in the colorless range (DEF) receive no appearance benefit from fluorescence, and the market penalizes these diamonds in terms of value depending on the strength of the fluorescence. Thus, a D color with Very Strong fluorescence would be heavily devalued, and that is therefore BAD.
GIA Diamond Fluorescence Comparisons
Diamond Fluorescence – Market Factors
The market has been historically rather fickle with regard to diamonds fluorescence. In the early days “Blue White” diamonds were highly prized. These were diamonds in the colorless range with strong blue fluorescence and no deleterious visual effects. The term became so pervasive in the marketing of diamonds that the Federal Trade Commission in 1938 felt compelled to pass regulations for the use of the term! The diamond trade also had a term for those “blue whites” that prominently display the problematic milky effect. They were referred to as “over blues” and that term is still used by members of the diamond industry.
One of the perceived negative aspects of diamond fluorescence is that an authentic gem could be confused with early simulants, some of which also had this property. With the advent of the commercial use of black lights in public places such as discothèques, consumers could become aware that their diamond was very different! That could be off-putting if you were not prepared for it.
As mentioned earlier, diamonds (particularly in the colorless range) are devalued on the market today based upon fluorescence levels. On the wholesale level diamonds are discounted by 15% or more on the basis of strong or very strong fluorescence. That lost value is compounded at the retail level. The penalty is further exacerbated in the second-hand market to the point that liquidity itself is compromised.
Over the years there have been temporary cycles of interest in highly fluorescent diamonds, but in the modern market many buyers are cautious about it. The market today seems to perceive fluorescence in a couple of distinctly negative ways. First, there is an assumption that the milky effect that is seen in some strongly fluorescent stones must be present to some extent in diamonds of lower strengths. And secondly, the purity of the diamond is suspect because of the presence of a “defect” causing the fluorescent effect. For these reasons and the others outlined above, A CUT ABOVE® Super Ideal Diamonds
are required to have negligible fluorescence.
Laboratory Reporting of Diamond Fluorescence
Diamond fluorescence is assessed in the lab with the aid of a set of fluorescent master stones and specific viewing angles. Interestingly, fluorescence can occur in localized zones within the diamond and can be more or less visible depending on the viewing angle. A vivid example of this phenomenon is the diamond pictured below. In this diamond the area within the stone that is emitting light is very localized and the strength of the fluorescence is highly directional.
Diamond Fluorescence - Directional Effects
Multiple areas of fluorescence are also possible within the same diamond and can display distinct colors or create a mixed color. Such diamonds are very rare.
It is important to note that the first graphic above (GIA Diamond Fluorescence Comparisons) illustrates the GIA reporting scale. American Gem Society Laboratories (AGSL) considers None and Faint together as “Negligible” since neither have any impact on visual appearance. Another distinction between the labs is that GIA determines a diamonds fluorescence in table-down position whereas AGSL uses the table-up view. More information on diamond grading can be found on our pages about the American Gem Society Laboratories
The biggest study ever undertaken to understand the visual effects of fluorescence on diamond was done by the GIA and published in Gems and Gemology in 1997. The study involved carefully selected sets of diamonds in various colors and degrees of fluorescence, and involved three groups of observers; laboratory graders, jewelry trade members, and average observers. The diamonds were observed in a variety of specific lighting environments. The results of the study suggest that diamond fluorescence has very little impact on visual appearance. In its conclusion the authors state "One interesting aspect of this study was that nontrade observers could not make meaningul distinctions. For this group, which would be considered most representative of the jewelry-buying public, fluorescence had no over-all effect on color appearance or transparency". Data from the trained observers indicated a mild positive impact color in the table-up view, and even smaller negative impact on transparency. It should be noted that “over blues”, very strong blue fluorescent diamonds displaying prominent milkiness, were not included in the test sets used in this study. For more information you can download the full GIA study titled A Contribution to Understanding the Effect of Blue Fluorescence on the Appearance of Diamonds
Fluorescence Assessment by Jewelers and Appraisers
As discussed above, even the top labs in the world differ on their methodology and reporting of fluorescence. However, in each lab the process of observing and reporting is highly controlled and consistent. But such is not the case out in the field where jewelers and appraisers use a wide variety of devices and techniques to assess fluorescence. These various instruments contain light sources that emit variable wavelengths of UV and VV stimulation resulting in a wide range of possible observations. Therefore, while a diamond may have a GIA report showing faint, or an AGS report showing negligible fluorescence, a gemologist in the field may very well assess it as medium or even strong.
“The variability in excitation wavelengths and bandwidths among commonly used lamps and LED UV sources demonstrates that the colors and intensities of observed fluorescence in a single sample can vary depending on the light source.”
“…small changes in excitation wavelength, even from a “pure” LWUV emission, can significantly affect the intensity and possibly the color of the fluorescence.”
“LED sources are far more constrained in emission bandwidth and purity, but they are manufactured in such a wide range of wavelengths that consistency among different products remains a problem.”
It is therefore advisable to rely on laboratory reporting for an accurate assessment of fluorescence. If a field test by a trade professional indicates a strong possibility that an error was made in the lab, a re-check by the lab should be requested.