Diamond Fluorescence

Guest written by Jason Shonson.

Diamond Fluorescence is the term used to describe how approximately one third of all diamonds appear to change color when subjected to ultraviolet light. Scientifically, this process occurs when a substance (in this case, diamonds) absorbs light of a different wavelength, then emits the light. Almost always, diamonds glow various shades of blue under short-wave ultraviolet light. Rarely, diamonds may glow yellow, white, green, orange, or red. Diamonds also fluoresce under X-ray radiation.

Diamonds exhibiting strong or very strong blue fluorescence often appear milky or hazy, even oily, in regular light. Those that fluoresce moderately blue rarely have a milky or hazy appearance, and those that fluoresce faintly or weakly blue never have a milky appearance. Diamonds can fluoresce on a level ranging from faintly, weakly, moderately, strongly, to very strongly. Of the one third of all diamonds that exhibit fluorescence, just over one third will glow faintly, and just under two thirds will glow moderately to very strongly.

Diamonds that fluoresce can appear to change color under ultraviolet light. For example, a yellow-tinted stone that fluoresces blue could appear less yellow when studied under a fluorescent bulb. A stone that fluoresces yellow may appear white under incandescent light, and only exhibit its yellowish tint under short-wave ultraviolet light.

As yellowish diamonds are not as highly valued as whiter stones, stones fluorescing yellow often command a lower price. If a yellowish stone fluoresces blue, however, its price may increase, as the yellowish tint will be more masked. Fluorescing blue is more helpful in creating better color appearance in stones graded "I" or "J" color than on more colorless stones graded "D," "E," and "F." Fluorescence can make lower colored diamonds appear more white or colorless than they truly are, and it rarely affects the brilliance or sparkle of the stones.

Studies have suggested that diamond dealers and consumers favor fluorescent diamonds over non-fluorescent ones. This holds true for lower color range diamonds and higher color range diamonds.

It is not desirable to purchase diamonds that fluoresce rare colors such as yellow or orange, unless the diamond's color matches the color of its fluorescence. Even diamonds that are colorless will appear of a lower color grade under ultraviolet light if they fluoresce yellow or orange.