Diamond Grading - Fluorescence
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.