Learn About Diamond Fluorescence
"Fluorescence is a luminescence that is mostly found as an optical phenomenon in cold bodies, in which the molecular absorption of a photon triggers the emission of another photon with a longer wavelength. The energy difference between the absorbed and emitted photons ends up as molecular vibrations or heat. Usually the absorbed photon is in the ultraviolet range, and the emitted light is in the visible range, but this depends on the absorbance curve and Stokes shift of the particular fluorophore. Fluorescence is named after the mineral fluorite, composed of calcium fluoride, which often exhibits this phenomenon."
http://en.wikipedia.org/wiki/Fluorescence
Quite often I find clients are far too concerned over a diamonds' fluorescence grade that appears on grading reports and diamond certificates from accredited and recognised diamond laboratories.
 Is Diamond Fluorescence Bad?
This is a matter of personal opinion.
Fluorescence may enhance the overall brilliance of a diamond and can be an asset.
The one major concern is that the fluorescence does not affect the transperancy of the diamond by creating an oily or milky bluish white film over the diamond.
This can be of particular concern with diamonds in the colour ranges D, Exceptional White+ to G, Rare White.
But there are many diamonds with faint to strong blue fluorescence that despite this "assumed handicap" that still have a very good lustre and are overlooked or rejected because of the fluorescence grade.
From a commercial point of view and pragmatic reasons most dealers avoid stocking strongly fluorescent D-G coloured diamonds because of the percieved social stigma they possess which might make them harder to sell.
However in contrast, diamonds with a colour grade between H to K that have fluorescence are more in demand simply because they look better and are lower in price. I call these "show for dough" or "bluff" stones.
The term bluff stone can refer to diamonds that appear better than they actually are, when compared to the details listed on the grading report or diamond certificate.
Even a non fluorescent diamond can look hazy,oily or milky.
Diamond Transparency
The only factor that is usually missing from all certificates or grading reports is the transperancy of a diamond. At present there has not been a set standard to grade the transperancy of a diamond so it cannot be quantified.
Therefore it is imperative that you judge the transperancy with your eyes rather than just relying on a fluorescence comment on a grading report or diamond certificate. You should compare the diamond with other diamonds under cool white light, incandescent (halogen) light and also daylight as this will illustrate a diamonds transperancy and light return.
A very good cut diamond with very good transperancy and an equally very good lustre will look great under all three forms of light mentioned, whether it is fluorescent or not.
It's worthy to note that some Japanese companies charge extra for fluorescent diamonds by justifying that the diamond posseses an added optical phenomena similar to asterism ( six ray star) or chatoyancy (cat's eye).
 Diamond Transparency
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Diamond Fluorescence