Fluorescence as a term means that a diamond will glow (in blue) under a UV lamp when illuminated. It was used originally in diamond certifications as additional information for identification, but eventually it began to affect the value of the diamonds

 

The impact of blue fluorescence on price depends on its noticeability.For some higher color stones, fluorescence gives the stone a milky white appearance, which greatly lowers value. In some instances, the fluorescence is hardly noticeable and has minimal impact on the stone's brilliance and value. Fluorescence often adds value to lower color stones as it gives the stones a whiter, brighter appearance. Some buyers regularly pay better prices for highly fluorescent "I" color and lower stones. Yellow fluorescence may require an additional 5 to 10 percent discount. Generally, the higher the quality and price per carat the more strong fluorescence lowers value.

Diamonds containing fluorescence fluctuate in prices roughly by the following:

For D to H colors	IF to VS	SI1 to I3
Very Strong Fluorescence	-8% to -12%	-3% to 0%
Strong Fluorescence	-5% to - 7%	-1% to 0%
Medium Fluorescence	-2% to - 4%	0%

 

For I to N colors	IF to VS	SI1 to I3
Very Strong Fluorescence	0% to 4%	0% to 4%
Strong Fluorescence	0% to 4%	0% to 4%
Medium Fluorescence	0% to 3%	0% to 3%
Slight to Faint	0% to 1%	0%

 

Diamond Clarity is a measure of a diamond's internal flaws and impurities. A diamond that is virtually free of interior or exterior inclusions (commonly reffered to as flaws) is of the highest quality, for nothing interferes with the passage of light through the diamond. To determine a diamond's clarity, it is viewed under 10x magnification by a trained gemologist. Small inclusions neither mar its beauty nor endanger its durability.