asscher cut diamond with bluefluorescence This article was written by the founder of rockdiamond.com, and edited for print and post by me.

The question is far more complicated than it might seem. A fair percentage of all diamonds mined exhibit fluorescence. I'd estimate the number at 15- 25%. Here's why.  Grading involves assessing desirability, or judging diamonds. Fluorescence is an aspect which works in concert with other physical characteristics with some unusual results.

How does florescence affect a diamond visually?

In general, here's the affect of GIA graded fluorescence on colorless, or near colorless diamonds:
 
Faint or Medium Blue fluorescence:
Can be noticed in a dark room when exposed to a UV light, but not under normal circumstances. Fluorescence may be slightly noticeable in direct sunlight or other bright lights.

Strong Blue fluorescence:
Reacts heavily under UV light in the dark. In 25% of these cases, the fluorescence may be noticeable under any light source (though not necessarily in a negative way).
Likely to react quite strongly in direct sunlight, or other bright light sources.



Exactly what should one expect to see?

In the photo below, I've photographed a parcel of Asscher Cut Diamonds- the two which exhibit fluorescence are on the top left hand corner.

Likely I did not need to tell you where the fluorescent stones are- it's so obvious in a dark room with an Ultra Violet Light. In the sun, a colorless or near colorless fluorescent diamond may actually look a bit blue- depending on the degree of fluorescence.

Will my fluorescence Diamond Appear Milky, or Hazy in the sun?

Again, we need to look at degree. On Faint, and Medium, we can pretty much rule out any milky effect- even in direct sunlight.

With Strong Blue, in a small percentage of stones you might get a hazy , or milky look.
This is rare, though.

Cushion cut diamond graded Strong Blue Fluorescence by the GIA