When you are planning to purchase a diamond it is important that you know how diamonds are classified. Today, the 4Cs of diamond quality is the universal method for assessing the quality of any diamond, anywhere in the world. Moreover, there's "Fifth C" : Certificate

Diamond Fluorescence is a highly debated topic in the diamond industry, but a 1997 study by the GIA suggests fluorescence makes very little difference to the appearance of the diamond. In the diamond industry, the existence of fluorescence in a diamond (caused by the natural mineral properties of the diamond), has caused many debates and has, in many instances, influenced the pricing of diamonds.

It is common to find that diamonds with colorless grades (D-E-F) or near colorless grades (G-H-I-J) are lower in price when they exhibit fluorescence and faint yellow grades (K-L-M) are higher in price when exhibiting fluorescence.

The “theory” has been that:

1) Fluorescence has a negative impact on colorless diamonds (making them appear cloudy).
2) A positive impact on faint yellows (blue fluorescence supposedly counter-balancing the yellow color and making the diamond appears whiter).