What You Don’t Know About Fluorescence
Can Be Costly!

By Antoinette Matlins, PG

If you have ever visited gem and mineral exhibits at natural science museums, such as the American Museum of Natural History in New York City, The Smithsonian in Washington, DC, or the Houston Museum of Natural Science in Houston, Texas, you will probably remember seeing a display of “common-looking” rocks in various shades of white, gray, black, and so on, which suddenly—and magically—changed into intense, psychedelic hues of red, orange, yellow, green, blue, and violet, with just the touch of a switch!

People wonder how this happens, and it is really fascinating. When we “see” color, we are seeing certain wavelengths of light that are in our “visible spectrum”—which includes red, orange, yellow, green, blue, and violet. There are also wavelengths of light that go beyond our visible spectrum, and so they are invisible to us. For example, ultraviolet (UV) wavelengths are beyond the violet end of our visible spectrum; they exist, but we can’t see them. At the other end of our visible spectrum, there are infrared wavelengths that are beyond the red, and also invisible to us. When some rocks—such as the “magical rocks” you may have seen in these museum exhibits—are viewed under invisible ultraviolet wavelengths, we suddenly see colors that were previously invisible to us! We call these fluorescent rocks, and the reaction you see is the result of switching the lighting from normal light to “black light,” a special type of light that produces ultraviolet radiation—those wavelengths that are beyond violet and invisible to the human eye. The strong ultraviolet radiation emitted from this special lamp enables us to see wavelengths that are normally invisible to us—hence the incredible colors that suddenly appear!

When you turn on the black light, the rock’s fluorescence is excited and you see those fabulous colors. When you turn it off, the rocks go back to their stable state, and you see their normal, ordinary colors again. In some rare cases, a few of the rocks will continue to glow for a short time. We call this continuous glow phosphorescence (some watches have phosphorescent dials, which “glow in the dark” for a brief time when you go from light into dark). But the fluorescent colors are seen only when exposed to ultraviolet radiation.

Some diamonds also fluoresce when exposed directly to ultraviolet wavelengths. If the diamond you are considering is accompanied by a diamond grading report issued by a respected gem-testing laboratory, such as the Gemological Institute of America (GIA), the report will indicate if the diamond shows any fluorescence, and the strength of its reaction when exposed to ultraviolet radiation. Here again, the fluorescent color of any diamond will only be seen when the stone is exposed to strong ultraviolet emissions.

If a diamond fluoresces, what color it fluoresces, and the strength of its fluorescence (faint, weak, moderate, strong, or very strong) is determined by viewing the diamond with a special lamp known as an ultraviolet lamp (UV lamp) which emits only ultraviolet radiation—the black light used by museums to show the fluorescent colors of the rocks in their fluorescent mineral collections is a type of UV lamp. A UV lamp is a standard piece of equipment used in gem-testing laboratories and by diamond dealers and knowledgeable jewelers.

Some diamonds fluoresce while others do not. It is estimated that over 40 percent of all diamonds fluoresce, and that of those, approximately 15 percent will have a strong or very strong fluorescent reaction to ultraviolet radiation. A diamond can fluoresce any color, but the overwhelming majority of colorless diamonds fluoresce blue (approximately 98 percent) with an extremely small percentage fluorescing yellow or any other color. In fact, fluorescent colors other than blue or yellow are so rare in diamonds that this can be an important “identifier” of your diamond!

Remember that whatever color a diamond fluoresces, the fluorescent color will only be seen when viewed directly beneath a UV lamp. In the case of colorless (white) diamonds, the stone is really colorless, and it will look colorless in normal indoor light or evening light. Nonetheless, if the diamond has an adequately strong reaction to UV radiation—“medium,” “strong,” or “very strong”—this property can affect the color you see in outdoor daylight and when viewed directly beneath a UV-emitting light source.

It is interesting to note if a diamond fluoresces, and what color it fluoresces, because there are varying wavelengths of ultraviolet radiation in the lighting around us and this can trigger a diamond’s fluorescence and make the color appear whiter, or less white. A diamond that fluoresces strong or very strong blue, for example, will appear whiter when seen outdoors, during daylight hours—sometimes several grades whiter—because the very strong ultraviolet radiation present outdoors will “excite” a strong fluorescent reaction in the diamond, causing it to emit some degree of blue, which then masks any faint yellow tint that might be present. Some colorless diamonds (often in the D- to F-color range) with strong or very strong blue fluorescence can actually look “light blue” in outdoor light! At one time, these diamonds were called “blue-white” diamonds, but this term should not be used today except in such cases where the stone actually shows a bluish hue.

Whatever fluorescent color a diamond may show outdoors during daylight hours, it is rarely seen when viewed indoors unless it is held very close to a window through which daylight is pouring in (a few inches from the pane of glass), or if viewed within a few inches of a daylight-type fluorescent light fixture. In some cases, a strong incandescent light can also trigger some degree of blue fluorescence. However, the impact of lighting in terms of triggering a fluorescent reaction becomes increasingly weaker the farther away one moves from the light source. So the color seen in indoor lighting environments will be the diamond’s inherent (true) body color, the color seen when the lighting lacks sufficient UV emission to trigger any fluorescent reaction.

Is Fluorescence Desirable or Undesirable?

When it comes to fluorescence, it is important to understand that as long as the true body color is indicated on the diamond report or by the seller, and you are paying an appropriate price for the diamond’s true color, it is neither a good thing or a bad thing; it is a matter of personal preference. Many people are fascinated by fluorescence and search for a fluorescent diamond. For some, the idea that a blue-fluorescent diamond will look whiter in outdoor daylight is a plus, an extra benefit. For others, yellow fluorescence is preferred because they are usually discounted as a result of a trade bias against them (often, unmerited), and some would rather pay less, knowing that any tint of yellow will only appear when seen outdoors during daylight, and that most of the time the diamond will be seen indoors or at night and be comparable in color to more expensive stones!

We want to emphasize again that whatever color a diamond fluoresces, and whatever impact it has on the color you see while it is fluorescing, the same stone will show its true body color when seen in most indoor environments or in evening light where no fluorescent reaction is being triggered.

To better understand fluorescence, let’s imagine you are looking at three diamonds with an inherent body color that has been graded “G-color.” One stone has no fluorescence, one has strong blue fluorescence, and one has strong yellow fluorescence. The stone with no fluorescence will look the same color indoors or outdoors. The blue-fluorescent diamond will appear G-color indoors, but will look whiter outdoors during daylight hours—it could appear “F-color,” or even “D-color” or “E-color,” depending upon variables affecting the intensity of the UV, such as time of day, altitude, the hemisphere you’re in, smog, and so on! The yellow-fluorescent diamond will appear G-color indoors, but will look more yellow outdoors during daylight, just how much so depending upon those variables mentioned above. The important thing to understand is that although the color may be whiter, or yellower, when viewed outdoors during daylight hours, all three diamonds will be the same color when seen indoors or in evening light, which is where most diamonds are seen, most of the time.

To ensure that the inherent body color—the stone’s true body color—is the grade shown on any diamond grading report, the stone should always be tested for fluorescence with an ultraviolet lamp prior to color grading. If the diamond fluoresces, the grader should use a UV-free light to color grade the diamond, or a UV-filter to screen out any UV emission that will “excite” a fluorescent reaction that will result in an erroneous grade.

Does Fluorescence Affect Value?

If the color grade represents the inherent body color of the diamond, fluorescence will have little, if any, impact on value. Historically, diamonds were color graded under lighting conditions in which the UV emissions were filtered out so that the grade did, in fact, represent the diamond’s true body color. In addition to the body color, the report would also indicate if the diamond fluoresced, and the intensity and color of the fluorescence. Thus, knowledgeable buyers often sought blue-fluorescent diamonds, knowing that they would look whiter in certain environments. At that time, diamonds with blue fluorescence were very desirable, offering a little bonus since they looked whiter in daylight! A blue-fluorescent diamond could even command a small premium over a nonfluorescent diamond.

In recent years, with the advent of fluorescent lighting coming into homes and offices, the lighting used to color grade diamonds changed because graders believed—incorrectly—that indoor “fluorescent” lighting produced UV emissions that were sufficiently strong to stimulate a fluorescent reaction. Thus it was concluded that the lighting used to color grade diamonds should also be UV-emitting. So the color grade shown on diamond reports for fluorescent diamonds changed from one that represented the stone’s inherent body color, to one that represented what the color appeared to be when its fluorescence was excited, believing that this grade better represented the color that would usually be seen, indoors as well as outdoors. Unfortunately, recent research has shown this is not the case because indoor lighting environments are virtually UV-free, and lack sufficient UV wavelengths to excite a diamond’s fluorescence (or to cause any health risks to people living and working indoors where fluorescent lighting is used).

Unfortunately, many laboratories do not yet realize that color grading diamonds with a UV-emitting light source can result in erroneous grades for some fluorescent diamonds. As a result, they continue to use UV-emitting lighting. In the case of diamonds that fluoresce blue—which, as explained, represents the vast majority of fluorescent diamonds—the stone will be overgraded, and if overgraded, it may also be overvalued. When overvalued, the greater the variance between the stone’s inherent body color and a “perceived” color (when fluorescing), the greater the variance in its value.

Generally speaking, if the strength of a diamond’s fluorescent reaction is “negligible,” “faint,” or “weak,” it will have no effect on the diamond’s color. But when more intense, this is not the case. A diamond that exhibits “medium” fluorescence can differ by one half to one full grade from its inherent body color; if strong, by one to two grades; if very strong, by three to four grades. If the color grade on the report represents a “perceived color” resulting from blue fluorescence, you may be paying more, possibly much more, than you would be paying if the report indicated the stone’s true body color, the actual color you will normally see. You want to enjoy seeing the color you pay for!

We cannot overemphasize this point. If a laboratory grades the color of a diamond using a UV-emitting light source, the color grade on the report will not represent the stone’s inherent body color—its “true” body color—but a “perceived” color, a color that will only be seen when its fluorescence is excited. If a diamond fluoresces blue, this means the grade will indicate a whiter color than the color that will normally be seen. If it fluoresces yellow, the grade will represent a color lower than what will normally be seen.

For this reason, we recommend that prior to buying a fluorescent diamond, you have it double-checked by a gemologist who uses a UV-filtered light source to color grade, so that you know if the grade shown on the report is its true body color, and if not, learn what it is, before you purchase it. If this is not possible, then we recommend looking at any fluorescent diamond in several different lighting environments to see if you can detect any color change. Check it in direct daylight and directly beneath a fluorescent light, and then in “normal” lighting—in the middle of a room or somewhere several feet from any light source. If the stone appears the same color in all lights, then its grade is probably accurate. If the stone looks less white when away from daylight or the fluorescent lamp, then it is probably overgraded. One way to help determine this is to compare the fluorescent diamond you are considering to another diamond of the “same” color grade that is not fluorescent, noting if they both appear the same color when you look at them in the middle of the room, away from daylight or any other light source.

The Accredited Gemologists Association (AGA) created a task force to investigate the issues of lighting and color grading diamonds and recently presented the latest scientific findings on this subject. It has recommended the use of UV-free lighting for color grading diamonds, to assure that grades indicate a stone’s true body color. We are optimistic that the AGA recommendations will be accepted by the trade, but for now we recommend taking extra precautions when buying diamonds with medium, strong, or very strong blue fluorescence unless the price is significantly below the price of another nonfluorescent diamond with the same color grade.

If you are considering the purchase of a beautiful fluorescent diamond, but are not sure if the price is reasonable, we recommend taking a “worst case” approach to trying to determine if the diamond is appropriately priced in terms of its true body color. For example, if you’re considering a one-carat diamond with “medium” blue fluorescence, that has a report indicating E-color, we would not pay the same price as we’d pay for a nonfluorescing diamond with an E-grade. Since “medium” fluorescence can affect the grade by one half to one full grade, we’d compare its price to the price of a nonfluorescing diamond that is graded F-color, add a little for the extra benefit of looking whiter outdoors, and see how that price compares to the price being asked for the stone. In this case, if a nonfluorescing E-color sold for $20,000, and a nonfluorescing F-color for $17,000, a realistic price would be somewhere around $18,000–$18,500. Here the price difference isn’t so great, and if the stone is really beautiful and you love its personality, you probably wouldn’t be making a mistake even if you paid more, but you shouldn’t pay the same price as you would for a nonfluorescing E-color diamond. However, if the E-color diamond had “very strong” fluorescence—which could affect the color grade by three to four grades, to an “H-color” or “I-color”—the cost difference is considerable. In such a case, you’d be wise to take the stone to an independent gemologist-appraiser to ascertain its true body color. If this isn’t possible, ask the price of a comparable, nonfluorescing I-color diamond, add a small premium for the benefit of looking a lot whiter outdoors, and see how this price compares to the price being asked for the stone you are considering. Using the same parameters as in the earlier example, an I-color diamond would be about $11,000, and after adding a small premium, figure a “fair price” to be around $12,000–$13,000. If this is close to the price being asked, the stone is probably priced appropriately. If the seller wants a lot more, our advice is to find another diamond!

We want to stress that if a diamond is properly priced, there is nothing wrong with buying a fluorescent diamond, whether it fluoresces blue or yellow, or any other color. Fluorescent diamonds are fascinating, distinctive, and have an allure of their own!