Diamond fluorescence is a form of illumination that is created when a diamond is exposed to Ultraviolet (UV) light. About a third of all diamonds will glow under ultraviolet light, usually a blue colour which may be noticeable under a black light or strong sunlight. According to the GIA, who reviewed a random sample of 26,010 natural diamonds, 65% of the sample had no fluorescence. Of the 35% that did have fluorescence, 97% had blue fluorescence of which 38% had faint blue fluorescence and 62% had fluorescence that ranged from medium to very strong blue. Other colours that diamonds can fluoresce are green, yellow, white and red but are very rare and are sometimes a combination of the colours such as blue-green or orange.
Some diamonds with “very strong” fluorescence can have a “milky” or “oily” look to them, but they are also very rare and are termed “over-blues.” Their study concluded that with the exception of “over-blues” and yellow fluorescent diamonds, fluorescence had little effect on transparency and that the strong and very strong blue fluorescent stones on average had better colour appearance than non-fluorescents. Since blue is a complimentary colour to yellow and can appear to cancel it out, strong blue fluorescence had especially better colour appearance with lower colour graded diamonds that have a slight yellowish tint such as “I” colour or “J” colour but had little effect on the more colourless “D” through “F” colour grades.
The GIA released the results of a study in 1997 that addressed the effect of blue fluorescence on the appearance of diamonds. Here are some of the conclusions of that study:
- In general, the level of fluorescence had no widely perceptible effect on the colour appearance of diamonds viewed table down (upside down).
- In the table up position, diamonds described as Strong or Very Strong fluorescence appeared to have better colour than less fluorescent stones.
- The effect of fluorescence on colour was most noticeable in the lower (I-J-K) colours.
- Non-trade observers in the test had more difficulty than those in the diamond trade in seeing meaningful distinctions of fluorescence on colour appearance or transparency.