Diamonds have many unique properties, including natural blue
fluorescence under any source of ultraviolet light.
One technique that gemologists
use to help identify and separate diamonds from the diamond look-alikes
(such as moissannite and cubic zirconia) is by putting the gem
under a UV lamp. If it fluoresces (glows) blue, it is almost
certainly a diamond. In fact, most diamonds have some degree
of fluorescence under natural light, which contains small amounts
of UV frequencies.
(However, it is important
to know that you cannot follow this logic in reverse and assume
that when something does NOT fluoresce it must not be a diamond.
Some diamonds do not have fluorescence so this logic will
How does fluorescence effect the beauty or value of a diamond?
The answer to that question depends on the color grade you are
First let's talk about
how it affects the beauty.
Actually, recent studies
by GIA have shown that fluorescence in any amount does not impact
the face up appearance of a diamond, except possibly those with
extreme fluoresence (beyond the standard Very Strong grade --
stones with extraordinary fluorescence are rare and were not
available for testing at the time). Therefore, it is only the
prevalent belief of the trade and consumers that causes
less demand for stones with strong fluorescence and brings the
Here is the prevalent belief system to help you understand how
fluorescence influences price:
If a diamond has a color
grade of J to M, a moderate amount of fluorescence will actually
make a diamond more attractive to most people. Slight to moderate
or even strong blue fluorescence in a stone with these color
grades actually helps cancel some of the yellow and makes it
However, for a diamond with very high color (such as a D to F
grade), fluorescence is thought to interfere with the flow of
light and make the diamond appear a little oily or murky. This
will not be true for most diamonds, but it is thought to be so
and you can buy if for less. We still feel a little hesitant
to recommend D to F color diamonds with strong fluorescence,
despite the recent research. This is just because of our many
years of training that we should avoid them. For this reason,
we still might recommend None or Slight flourescence in colorless
diamonds (D to F color grades).
In grades inbetween
those above (G, H and I), it might be better to stay away from
Strong fluorescence for safety's sake, while moderate fluorescence
might actually improve the color appearance in our opinion.
Adjustments for stones
of each level of fluorescence are too difficult to explain here. If you want to learn how to calculate a price for any diamond you want, please see my easy, fast, step-by-step Diamond Pricing Tutorial page.
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