Diamond Fluorescence
Some gem-quality diamonds emit colored light when they're exposed to ultraviolet radiation, such as that found in daylight.
If a diamond has strong fluorescence, it can look cloudy or oily. This diminishes the brilliance of the diamond, and so it usually decreases the value of high quality diamonds. A rare type of fluorescence called overblue produces a particularly cloudy effect.
Lower quality diamonds can actually benefit from fluorescence. Blue fluorescence can balance out the yellow in a lower color grade diamond, making it look whiter and clearer in daylight.
Facts about Fluorescence from the GIA
Fluorescence is fairly rare. 65% of diamonds have none.
Strongly blue fluorescent diamonds possessed better color grade in the face-up position.
No fluorescent effect was observed in the table-down position, which is how labs color grade diamonds.
Color fluorescence was more apparent in diamonds grades I through K.
Strongly or very strongly fluorescent diamonds were considered to have better color appearance than less fluorescent stones.
Blue fluorescence had little effect on transparency.
Overblues are rare, so they were not included in the GIA study.
The GIA conclusion: fluorescent diamonds are stigmatized needlessly, which puts a negative impact on their salability.
Fun factoid: The famous Hope Diamond fluoresces an extremely unusual red-an occurrence in blue diamonds - and this has influenced the legend of its curse.
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