Fluorescence is a naturally occurring phenomenon in diamonds where a diamond will illuminate when exposed to UV light. Diamond fluorescence is graded from none to very strong and can vary in color. The most common color for fluorescence is blue, but it can also be yellow. Fluorescence does not usually affect a diamond’s sparkle, and in lower color graded diamonds (I, J, K), fluorescence can make a diamond look whiter or more colorless when in UV light. Blue and yellow are complementary colors therefore diamonds with slight yellow body color benefit from fluorescence because it whitens the body color. Approximately one third of all diamonds in the market exhibit some level of fluorescence, so it is very common and bares no harm to the beauty of the diamond.
The benefit of fluorescence does not apply to all loose diamonds in the normal D-Z color range; in fact, fluorescence is not desirable in the higher color ranges D, E, F, also known as colorless diamonds. The whiter a diamond, the more fluorescence can be detrimental to the beauty of the stone. Fluorescence has the potential to make the body color of a colorless diamond look blue and/or milky because there is no yellow body color to combat the color. The term milky is exactly what you think it would look like; the stone becomes cloudy and can give off an oily appearance.
The effect of fluorescence on a diamond should be judged individually since color is not the only feature that is affected by fluorescence. Different diamond cuts such as oval and cushion show body color more easily at higher color grades, so fluorescence can be beneficial in some cuts over others. Since fluorescence ranges in strength, typically faint-to-medium has no detrimental effects even on high color grade diamonds. As a gemologist, I always consider the effects of fluorescence when selecting a diamond for a customer.
It is very common for fancy yellow diamonds to also have fluorescence. Since yellow is the most common color for fancy colored diamonds, I will only discuss fluorescence in fancy yellow diamonds. Blue is still the most common color of fluorescence in fancy yellow diamonds and the effects are similar to that of a white diamond. The stronger the fluorescence, the lighter the stone will appear when exposed to UV light. Unlike diamonds in the mid-to-low color ranges of the normal white diamond D-Z color range which benefit from fluorescence, fancy color diamonds hold their value primarily for the intensity of their color. The stronger the fancy color grade (fancy –to-fancy vivid), the more comfortable I am with some presence of blue fluorescence. Strong blue fluorescence in a light yellow-to-fancy light yellow diamond can, however, soften the body color further when in UV light. Unlike a white diamond where any color of fluorescence that is not blue is strongly not recommended, a fancy color of the same hue as the fluorescence will make the color more intense! Although yellow fluorescence is more rare than blue, a fancy yellow diamond with yellow fluorescence has the potential to have a more intense color at a lighter fancy color grade!