Apart from the 4Cs (cut, colour, clarity and carat), diamond graders use fluorescence to assist in distinguishing one diamond from another. Fluorescence refers to the way a diamond glows under ultraviolet light. Diamond grading reports display the fluorescence of a diamond in the following ways:
- None – no glow under a UV light
- Faint – faint glow under UV light
- Medium – obvious glow under UV light
- Strong or Very Strong – profound glow under UV light
Diamond fluorescence is a natural effect that appears in approximately 30% of all diamonds and there are certain misconceptions that have arisen over the years. Fluorescence is most often in a bluish hue but it can also be in other colours, namely yellow, orange, red, white or green. In about 0.2% of diamonds, fluorescence can cause a cloudy or oily look which can be considered undesirable. In most other cases, a medium or strong blue fluorescence can give diamonds a whiter appearance.
The Gemmological Association of America (GIA) is one of the most popular and respected diamond laboratories in the world. Here is their advice on fluorescence:
“GIA studies show that, for the overwhelming majority of diamonds, the strength of fluorescence has no widely noticeable effect on appearance. In many instances, observers prefer the appearance of diamonds that have medium to strong fluorescence. In rare cases, some diamonds with extremely strong fluorescence may appear hazy or oily; fewer than 0.2% of the fluorescent diamonds submitted to GIA exhibit this effect.”