Fluorescence

Blue fluorescence, the impact on color and value ...

If a diamond reflects a bluish glow when excited with UV light the stone is considered "fluorescent blue." The intensity of fluorescence is described by the GIA as follows:

None [NN]; Faint [FT]; Medium [MED]; Strong [STB]; and Very Strong [VSB].

Faint and medium fluorescence are typically not detectable to the naked eye and may be viewed simply as a characteristic of any diamond.

Faint fluorescence does not influence value while medium fluorescence in colors H and higher should be discounted a couple of percent; however, when considering 'I' color or lower, faint or medium blue fluorescence actually enhances the color and makes these diamonds a few percentage points more valuable than equivalent colors without fluorescence.

"Strong" and "Very Strong" blue fluorescence can often be detected with the naked eye and can cause a diamond to appear less brilliant and even milky.  As a general rule these stones should be avoided.

In rare instances "strong blue" fluorescence may have a minimal effect on brilliance; however, the mere presence of  STB or VSB on a certificate will lower the value by as little as 10% (if the stone remains bright) and as much as 90% (if the diamond has a "milky" appearance).