Roughly one third of diamonds available
in the market fluoresce, like the fluorescent minerals you have
seen in natural history museums, or the novelty shop toys under
the black (UV) light. The effect is like a white shirt in a nightclub.
Fluoro can be faint to very strong, and the most common fluorescent
color is blue. As blue is the complimentary color to yellow, the
most common tinted color in diamonds, blue fluorescence can make
yellowish diamonds look white or colorless.
A GIA survey found that fluorescent
diamonds were favored over non fluoro stones, especially in lower
colors, but even in the higher colors (D, E and F) which are often
discounted by the trade. Many years ago colorless fluorescent
diamonds were highly prized and refferred to as "blue-white".
But sales people used the term too loosely for any diamond with
fluorescence; "blue-white" usage was outlawed by US
trade practices laws.
One "for" argument for discounting fluorescent diamonds
is because the GIA lab grading lights emit a small amount of ultra
violet light; fluorescent diamonds might be assigned a better
color grade. The ‘against’ argument, although the
GIA Gem Trade Lab has not openly discussed the issue, is that
UV light is almost always present in viewing environments, so
why not grade color in realistic lighting?
But the most likely reason for fluoro diamond discounting is because
of the sad fact that many jewelry salespeople are not able to
explain complex phenomena like fluoro; a Fluoro (or any comments)
written on a report makes the diamond harder to sell = worth less!
Some diamonds have extremely strong fluorescence and appear oily
or cloudy. This is BAD. But the GIA study found them to be very
rare; they were unable to find enough cloudy stones from the 26,010
samples they used.
Rarely diamonds fluoresce another color like yellow or orange.
Do not buy them unless the diamond concerned is a fancy color
of the same hue as the fluorescence (which will make it more intense).
White diamonds with yellow or orange fluoro will appear to be
a lower color when seen in light with a UV component.
When the UV light is turned off, fluorescence ceases instantly,
but some stones continue to phosphoresce for a little while.
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