Diamonds have many unique properties, including natural fluorescence under ultraviolet light. One technique that jewellers use to help identify and separate diamonds from the diamond look-alikes (such as moisenite and cubic zirconia) is by putting the gem under a UV lamp. If it fluoresces (glows) blue, it is almost certainly a diamond. In fact, about 30% of diamonds have some degree of fluorescence under UV light (However, it is important to know that you cannot follow this logic in reverse and assume that when something does NOT fluoresce it must not be a diamond. Some diamonds do not have fluorescence so this will logic will not work.) Fluorescence usually appears blue, but can be yellow, green, or chalky white also.
How does fluorescence effect the beauty or value of a diamond? The answer to that question depends on the color grade you are buying. First let’s talk about how it affects the beauty. Actually, recent studies by GIA have shown that fluorescence in any amount does not impact the face up appearance of a diamond, except possibly those with extreme fluoresence. Therefore, it is only the erroneous belief of the trade and consumers that causes less demand for stones with fluorescence.
Here is the prevalent belief system to help you understand how fluorescence influences price:
If a diamond has a color grade of J to M, slight to moderate or even strong blue fluorescence in a stone with these color grades actually helps cancel some of the yellow and makes it looks whiter. No price difference occurs.
However, for a diamond with very high color (such as a D to F grade), fluorescence is thought to interfere with the flow of light and can make the diamond appear a little oily or murky. For this reason, we still might recommend None or Slight flourescence in colorless diamonds (D to F color grades). A a small discount (5-10%) may be given depending on the stone’s appearance. In grades inbetween those above (G, H and I), moderate fluorescence might actually impove the color appearance but not the value.