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
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
Listed below is GIA's list of abbreviations for strength
of fluorescence as well as their meanings:
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.
Faint or Faint Blue Fluorescence
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
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.