Between 25% and 35% of diamonds possess some fluorescence. The degree of it is described as None, Faint, Medium, Strong or Very Strong. Fluorescence is an emission of light by substance that has absorbed light or other electromagnetic radiation. Fluorescent materials stop to glow when the radiation source ceases its power. Some irregularities in the diamond’s atomic structure cause this fluorescence.
It is assumed that fluorescence does affect the diamond grades, but GIA grading is under no circumstances influenced by fluorescence due to properly controlled lighting conditions, which eliminate this influence. But the perception of the colour might be influenced by fluorescence when the diamonds are viewed under certain types of lighting.
On 16th November of 2017 (Sotheby’s Auction) the sale of the largest fancy intense pink diamond has failed due to its possession of strong blue fluorescence, as the private clients who were interested in this stone were very particular in regards with such degree of fluorescence.
But looking on the bright side of this quality, occasionally, I do have requests from my private clients, mostly from USA, who appreciate the positive side of fluorescence, amused by this quality and find these diamonds as fun to possess.