why do these have to be so complicated? 

Hi diamondnewbie - those are great questions!

I'm going to add a:     so he'll be more likely to see them!

Hi David. Thanks for all those answers. If you have any pics of diamonds with Yellow or White fluorescence on your webpage, pls post the link for us.. Love to see the comparison with the blue fluorescence...Or, maybe you've done this already?

      BTW... I think you should write a book on "User's guide to diamonds". 

OF COURSE SHE WOULD!   Just to be even cooler than all the other diamonds. 

Great question, Sarahbelle!

Of course, the answer is not a simple one.  For example, it really depends on the type of Diamond we are talking about.
Let's talk about value, the first part of your question:
If we're talking about a very high quality diamond in terms of color-that is to say D-E-F-G colors in the colorless range, or for talking fancy yellows, the grades of Fancy Yellow, Fancy Intense Yellow, and Fancy Vivid Yellow.
In these extreme grades, the prices are exponentially higher than the lower color grades.  So, if someone's paying that kind of a price, generally they won't want fluorescence.  Consequently, in these premium priced colors, medium or strong fluorescence may negatively impact the price.
If we're comparing D/ Internally Flawless diamonds of comparable cut quality and size, if one has medium or strong fluorescence, it will trade for less than the other.

Now, let's consider a J color in terms of fluorescence and its effect on value.
If a  D/ Internally Flawless  is $20,000 per carat ( it is in about that in one carat size), a comperably cut J/ Internally Flawless will be about $6000 per carat.
Generally, this is a less discriminating buyer.  Add to this the fact that in terms of the way the diamond looks, fluorescence can be a positive in the J color.
The result is that medium or even strong blue fluorescence may have absolutely no effect on the price of the J/ Internally flawless


Which kind of answers, the second part of your question: in most cases there is actually no reason to avoid fluorescence... in fact, some buyers seek out this characteristic.

What's FDY?

Green fluorescence are extremely rare! That is a collector's stone.
 

For colorless diamonds. I prefer them to be inert or with faint. For near colorless, I would like to see inert or faint for G but faint for H and at least Medium for I color and a strong for J. J color diamond with strong blue fluorescenece will appear D colorless face up! Now that increases it's value.

However. H color stones or higher will have a discounted price if they have any degree of fluorescence.

White fluorescence does exist. it's not all that nice when I've seen it though....

In terms of Blue Diamonds with Blue fluorescence....Good question!
There are so few blues that it is not easy to make generalizations...I have noticed a lot of pinks have Blue fluorescence.
In these ultra rare colors, the presence of fluorescence has little impact on price.

Most likely a reflection from the UV light.

Sometimes I have to have a strong blue stone to use form comparison because the reflection of the UV light tricks the eyes a bit.....

 Fluorescence are caused by impurities, crystal lattice irregularities or both. Most Type IIa or Golconda diamond does not fluoresce. These are the purest type of diamonds. A diamond that you cannot say or see anything bad.