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Some gem-quality diamonds fluoresce: they emit light when exposed to long-wave ultraviolet light. How does this fact affect a diamond's appearance and value?

In the past, some people in the diamond trade have considered moderate to strong fluorescence as a negative value factor for fine diamonds and a positive value factor for diamonds with a lower body color. Why? The trade perceives diamonds without fluorescence as "more pure" than diamonds with it. There is a perceived rarity for diamonds of fine color without fluorescence.

Fine quality diamonds with strong fluorescence may be undervalued because rare extremely strongly fluorescent diamonds known as "overblues" have a visible haziness that makes them appear almost cloudy in light with strong ultraviolet content. Dealers have theorized that strong fluorescence may affect apparent clarity.

At the same time, strongly fluorescent diamonds with a yellowish body color have long been considered to appear to have a better color because the blue of the fluorescence makes them appear more white in sunlight, which is a source of ultraviolet light. Lower-color diamonds with strong fluorescence sometimes command a premium.

The Gemological Institute of America's Gem Trade Laboratory, the country's leading diamond grading lab, lists fluorescence as an identifying characteristic, not a grading factor. And based on a GIA GTL random sample of data for 26,000 diamonds, diamonds with fluorescence are more rare than nonfluorescent stones: 65 percent of diamonds have no reported fluorescence.

Listed below is GIA's list of abbreviations for strength of fluorescence as well as their meanings:

     F or FB
No Fluorescence
Faint or Faint Blue Fluorescence
Slight Fluorescence
Medium Fluorescence
Strong Fluorescence
Extreme Fluorescence
To help clear up some of the confusion around fluorescence, GIA commissioned a study on the impact of fluorescence on the appearance of diamonds and published the results in its professional gemology journal, Gems & Gemology in the Winter 1997 issue.

The fluorescence study compared sets of round brilliant diamonds with a range of color grades in different controlled lighting conditions. The diamonds were evaluated by gemologists, dealers, and untrained observers.

The results? Untrained observers could not distinguish any affects of fluorescence. Even trained observers did not consistently agree. In general, strongly blue fluorescent diamonds were judged to have a better color grade in the face up position. (No affect of fluorescence was observed in the table-down position, which is how diamonds are color graded by laboratories.) No relationship between fluorescence and transparency was apparent. The effect of fluorescence on color was most notable in grades I through K.

The conclusion? "In the table up position (as is commonly encountered in jewelry), diamonds described as strongly or very strongly fluorescent were, on average, reported as having a better color appearance than less fluorescent stones. In this study, blue fluorescence was found to have even less effect on transparency. These observations confirm GIA GTL's experience grading millions of diamonds over the decades."

The study seems to indicate that over the years, the trade has unfairly stigmatized diamonds with strong fluorescence. And these diamonds are often available at a discount because of trade perceptions of possible negative impact of fluorescence on the salability of stones.

Because "overblues" are so rare, none of these diamonds were included in the GIA study. Overblues clearly do have a different appearance than ordinary strongly fluorescent diamonds. If you are ever in the Smithsonian institution in Washington D.C., you can see the most famous example of an overblue: the 127 carat Portuguese Diamond. While you are there, make sure to visit the famous Hope Diamond, which owes some of its legendary curse to the fact that it fluoresces an extremely unusual red, which is only known to happen in blue diamonds.