About 50 % of all diamonds possess the property of emitting a light blue or, rarely, a light green, yellow or reddish fluorescence. It is induced by radiation enriched with ultra-violet components and is visible for the duration of the radiation. An occasional persistence of light emission is termed phosphorescence. In most diamonds, however, the fluorescence is so slight that it can only be evoked by direct radiation with concentrated short or long wave ultraviolet light. In much rarer cases the blue fluorescence is evoked even by the concentration of ultra-violet rays in daylight.
When blue fluorescence can be observed in colorless diamonds even in daylight, these diamonds are additionally described as JAGER, thereby indicating their property of fluorescence.
Diamonds with slight to pronounced yellow tints can likewise show a more or less strong blue fluorescence; it creates in such stones a "petrol-colored" effect. These diamonds are described as petrol-colored or over-blue. The term Premier is also common in the trade, since many of the diamonds from the Premier mine exhibit this property.
| [ color-Grade] | Description of any Fluorescence present to supplement the color-Grade |
| Exceptional white Rare white |
Jager |
| White Slightly tinted white Tinted white Tinted |
Premier or petrol-colored or over-blue |
The property of fluorescence can be clearly seen with a long-wave ultra-violet lamp (366 nm). For this purpose the examination is carried out in a darkened room, the diamonds being laid on a black, non-fluorescent surface. If they are fluorescent, the stones glow in a light blue color. Diamonds which fluorescence even in daylight simulate a "better" color in it than in artificial light. In daylight a fluorescent diamond may appear to be a RARE WHITE; when examined with a constant light-source it may perhaps prove to be only SLIGHTLY TINTED WHITE. The correct description of this stone would read:
SLIGHTLY TINTED WHITE - strong blue fluorescence.
The body color observed in normal light determines the color-grade of fluorescent diamonds.
| The influence of fluorescence on price
is variable. Whilst fluorescent diamonds are much sought after
in the American. diamond market and are therefore five to ten
per cent dearer than diamonds without this fluorescence
property, buying interests in Germany lie, rather, in the
opposite direction. Strongly fluorescent diamonds have only a
few admirers. In general this property is of no importance. But when, in a piece of jeweler with several fairly large diamonds, some stones are obviously fluorescent, it can happen that in sunshine with concentrated ultra-violet rays the property of fluorescence becomes apparent and the stones affected stand out because of their petrol-colored appearance. |
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For ensuring identification every diamond's fluorescence should be observed in long-wave ultra-violet light at 366 nm and described. CIBJO has recently assembled a comparison set with reference stones, which represent typical degrees of fluorescence. They are described as follows:
nil - slight - medium - strong.
| Comparing diamonds of different hue | Summary of instructions on practical color grading |