The Issue of Diamond Fluorescence

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Home Gemological Information Diamonds The Issue of Diamond Fluorescence
 


Diamonds with faint, medium, strong and very strong blue fluorescence
as seen under ultraviolet light

Fluorescence occurs in some diamonds when excited by ultra-violet light, causing them to glow blue, though some may glow yellow and a few may glow white, greenish blue, green or pink. It is estimated that about one-fifth of all diamonds will fluoresce in long-wave UV light, though to different degrees.

Fluorescence can have a positive or a negative impact on the appearance of a diamond. In general, blue fluorescence enhances the “whiteness” of a diamond and is desirable up to the point that it causes a greasy or fuzzy appearance in the stone—where the impact becomes negative. Yellow fluorescence is always undesirable in white diamonds, but can have a positive impact on fancy yellow color diamonds.

Early on, GIA’s Founding Father, Robert Shipley noted that fluorescence was, in fact, a favorable feature in diamonds. He was quoted in Gems & Gemology, Vol. 5, No 9, p 398:

“It is important to stress the fact that fluorescence in diamonds is not a defect or an unfavorable quality. On the contrary, it is distinctly favorable. This is because in a paper of diamonds purchased by a jeweler in artificial light  which is a more reliable light for buying diamonds, the bluish fluorescent diamonds are the most desirable, since they are of superior quality in daylight and especially so in sunlight or near sunlight. Similarly, the layman who buys a [bluish] fluorescent diamond obtains one which has the same superior quality, and also a diamond which has a distinctive quality not possessed by every diamond.”

Several studies have demonstrated that blue fluorescence can improve the perceived body color of a slightly tinted diamond by one, two, three, and sometimes as much as four color-grades! However, for years all major diamond grading labs used special fluorescent grading tubes manufactured by Verilux that had most of the UV component filtered out. Diamonds were graded on a tray at the bottom of the Diamond Lite grading box, about 4 inches below the light source. At this distance, some (though not all) UV component in the light was dissipated. Diamonds were graded by their true body color only, without undue influence from fluorescence. The fluorescence was assessed separately, in a special “dark box” fitted with long-wave and short-wave ultra-violet tubes.

All was well until the Verilux Company stopped producing the UV-filtered lamps! As it came time to replace old or burnt-out tubes in the Diamond Lite grading boxes, labs purchased whatever tubes were available on the market. Gradually, the diamond-grading environment changed and Fluorescence replaced Cut as the dirty little secret in the diamond trade. It was reasoned that since so much indoor lighting was by then being produced by fluorescent tubes, the ambient light in most environments would contain ultra-violet light. Why not grade diamonds in the same light in which they would most often be seen?

Inexplicably, at around the same time grading distance from the light source was cut in half, from 4 inches to 2 inches or less, where the ultra-violet component of the light is strongest. Graders were taught to hold the diamonds right up to the fluorescent tube!

Make no mistake-blue fluorescence is usually a favorable feature in a diamond. The problem arises when such a diamond is graded while “excited” by ultra-violet light. It is now common to see a diamond with a color grade of “F” in unfiltered light suddenly become an “H” when the UV filter is put in place.

GIA and AGS are the two laboratories that certify sets of diamond “masters” used as comparison stones in color grading. Both have always mandated that the master stones be inert to ultra-violet light. So, while the diamond being graded may or may not be fluorescent, the master diamonds that set the grade will not fluoresce.

There is one other problem regarding fluorescence—the issue of standards. There are no industry standards of comparison for fluorescence strength. It is common to see diamonds these days with lab certificates stating fluorescence as “None” or “Negligible” that, when placed in the UV box, show “Faint” or “Medium” fluorescence. What is the meaning of “None”? Does it mean no discernable fluorescence in the UV box, or does it mean “less than Faint” fluorescence?

In testing these stones in the Diamond Lite grading box with and without the UV filter in place, some diamonds with as little as “Faint” fluorescence can improve by one grade when the UV filter is removed. With some faintly fluorescent diamond improving by one grade in the grading box, how can a buyer know whether a diamond with a grade of “None” was or was not in an excited fluorescent state at the time the color grade was assigned?

Testing of indoor light by members of the Accredited Gemologists Association in February, 2008 shows that there is very little ultra-violet component in most ambient light—even from fluorescent bulbs. The effects of distance from the light source and of “scattering” of light dilute the UV component. At normal working distances, fluorescent light bulbs cannot excite fluorescence in most diamonds. If a buyer purchases a fluorescent diamond with a body color of “H” or “I” in UV-filtered light and the diamond improves to an “F” or “G” outdoors in UV-rich sunlight, the fluorescence can be perceived as a benefit. If, however he purchases a diamond graded “F” or “G” in its excited state but displays its true body color of “H” or “I” in most indoor lighting conditions, then that is decidedly not a benefit.

There should be no problem regarding fluorescence in diamonds, once it is understood. My advice for diamond buyers is before purchase, have the diamond graded by a gemologist equipped to filter out ultra-violet light from the grading tube. This will let you make meaningful comparisons of diamond prices based on the diamond’s inherent body color—not an artificially induced color.