Fluorescence is the emission of bluish light emitted by about one third of gem-quality diamonds when exposed to long-wave ultraviolet light. The intensity may vary from faint to very strong. Under normal lighting conditions, this property of fluorescence is not detectable. However, a naturally fluorescent diamond tends to emit a soft colored glow when held under an ultraviolet lamp or “black light". Fluorescence is not harmful to the diamond or to the wearer. Most commonly diamonds show bluish light while exhibiting fluorescence, but can also show shades of yellow or white depending on the inclusions in the diamond crystal matrix. Fluorescence is described in the range from none (or inert), faint, negligible, medium, strong and very strong.
Fluorescence in diamonds can have both favorable and unfavorable effects. For example, diamonds in the color range from I to N with medium to strong blue fluorescence can appear more colorless, which is an obvious advantage. The bluish emission masks the very faint yellowish body color of these diamonds. Diamonds with very strong fluorescence can exhibit an oily or milky appearance, even under incandescent lighting. However, according to a GIA study, the average observer was not able to detect any difference in color or transparency when viewing diamonds with fluorescence.
Fluorescence of diamonds is often used to detect fakes. Gemologists identify and separate diamonds from the diamond look-alikes (such as moissannite and cubic zirconium) by viewing the gem under a UV lamp. If it emits blue light, it is almost certainly a diamond. In fact, most diamonds have some degree of fluorescence under natural light consisting of some UV frequencies. But the reverse cannot be stated true, that is, if a suspected diamond does not show fluorescence, it cannot be made certain that it is not a diamond.
For a diamond with color range from D to F, fluorescence is thought to interfere with the flow of light and make the diamond appear a little oily or murky. This may not hold true for most diamonds. But diamonds in the I to N range get a better color grade if they exhibit fluorescence.