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We see a note on fluorescence in most grading reports. What exactly does it mean?


To detect fluorescence, the best way is to observe the diamond under shortwave UV light. The 4 diamonds above have strong, medium, faint and no fluorescence in order. As can be seen, the strength of fluorescence is very obvious with the UV light on.

What causes Fluorescence?

Fluorescence is caused by the interaction between light energy and the atoms of the diamond.

How does it happen?

When carbon was subjected to the earth's pressure and heat about 120-200 kilometres below the surface of the earth, it crystalised over time to form diamonds. This usually occured where ancient volcanic sites existed because heat is required for crystalisation process. If there were traces of the mineral boron present in the earth as this crystallisation process was occurring the diamond will fluoresce. The various colors that diamonds can fluoresce are blue, yellow, green, orange and white. However, blue is the most common.

Do all diamonds fluoresce?

If there were no boron present during the crystalisation process, the diamond will not fluoresce. Only about 50% of diamonds fluoresce, and only about 10% have fluoresence that is strong enough to be noticeable outside lab conditions.

How is fluorescence graded?

Fluorescence is observed under shortwave UV lighting and given a grade of "Strong", "Medium", "Faint" or "None".

Is fluorescence good or bad?

Fluorescence can be good, or bad, depending on the colour of the diamond.

As a rule of thumb, better colours ("H" and better) do not go well with fluorescence. Usually, the fluorescence is not noticeable. However, if the fluoresence gets too strong, the diamond will appear milky or oily. So, do examine such a diamond carefully before making the purchase. In any case, it is important to know that the presence of fluoresence does lower the value of diamonds with better colour, just so you will not get ripped off.

However, for poorer coloured diamonds (worse than "H"), the presence of fluoresence improve the perceived colour of the diamond. This is because the blue hue offsets the yellow tint, making the diamond appear whiter than it actually is. This actually increases the value of the diamond. Of course, do not buy a diamond that fluoresce so much that it looks milky, in spite of the colour improvement.

Effect of fluorescence on price

Generally, the higher the quality of a diamond in terms of clarity and colour, its value will drop more with the presence fluoresence. Rappaport published a chart of the relationship as follows:


  Blue Approximate % Changes from non-fluorescents
Colours Fluorescence IF-VVS VS SI-I3
DE Very Strong -10 to -15% -6 to -10% 0 to -3%
DE Strong -7 to -10% -3 to -5% 0 to -1%
DE Medium -3 to -7% -1 to -2% 0%
DE Faint 0% 0% 0%
FGH Very Strong -7 to -10% -3 to -5% 0%
FGH Strong -5 to -7% -2 to -3% 0%
FGH Medium -1 to -3% 0 to -2% 0%
FGH Faint 0% 0% 0%
IJK Very Strong 0 to +3% 0 to +3% 0 to +3%
IJK Strong 0 to +2% 0 to +2% 1 to +3%
IJK Medium 1 to +2% 1 to +2% 1 to +2%
IJK Faint 0% 0% 0%
LMN Very Strong 0 to +4% 0 to +4% 0 to +3%
LMN Strong 1 to +4% 1 to +4% 1 to +3%
LMN Medium 1 to +2% 1 to +2% 1 to +2%
LMN Faint 0% 0% 0%


Next: Some tips on Colour...