When diamonds are exposed to ultraviolet light (uv) they can emit visible light. The resulting light can be of almost any colour, but most commonly it is blue. This phenomenon is very common in diamonds as well as in other gemstones.
About 40% of natural diamonds have some kind of fluorescence varying from faint to very strong. The different grades of fluorescence in diamonds are None, Faint, Medium, Strong and Very Strong. In about 10% of the diamonds the fluorescence is strong enough to produce a visible difference in the colour.
Is fluorescence good or bad?
About 90-95% of slightly tinted diamonds in the D to Z scale are from nature very light yellow. When daylight which contains UV rays start reflecting on the diamond the fluorescence effect starts emitting a blue light which in turn combines with the light yellow colour of the diamond and makes the diamond look whiter – This is called the white face effect of fluorescence.
Sometimes diamonds can emit a yellow fluorescence, in that case it negatively affects the colour of diamond. From time to time a Very Strong blue fluorescent diamond can have a hazy or oily effect. This negatively affects the sparkle of the diamond, in the trade this effect is called overblue, it needs to be mentioned that overblues are very rare and heavily discounted in the trade.
Both yellow fluorescence and overblues are very rare and occur in less than 1% of all fluorescent diamonds.
Fluorescence in Fancy Coloured Diamonds
Fluorescence in fancy coloured diamonds can have a much bigger variation of colours than in white diamonds, orange, white, yellow and green fluorescence are common in these types of stones. Fluorescence is usually not regarded as having a significant impact on the value in fancy colours.