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Fluorescence is the emission of visible light by a material such as diamond when it is stimulated by another, higher-energy form of radiation, such as X-rays or ultraviolet radiation. The emission of light stops when the energy source causing it is removed. Invisible trace impurities, such as nitrogen, can create submicroscopic structures in diamonds that enable the movement of electrons, which produces fluorescent emissions.
Although the ultraviolet unit commonly used to observe fluorescence in gems has two types of UV radiation, referred to as long-wave (a component of daylight) and short-wave UV radiation, the fluorescence discussed in the diamond trade (and described on a diamond grading report) is the response to long-wave UV.
At the GIA Gem Laboratory (GIA GTL), the standard procedure for observing UV fluorescence involves use of a long-wave UV lamp in a darkened viewing environment. Factors such as the distances and viewing angles between the UV lamp, the diamond, and the observer are specified to maintain consistency between observers. A set of reference diamonds is used to establish the intensity of the fluorescence exhibited by a stone within the ranges of none, faint, medium, strong and very strong. A comparison process similar to that used for color grading (with the D-to-Z scale) is also used to arrive at the fluorescence description stated on GIA GTL diamond grading reports. The fluorescence entry on a diamond grading report describes an identifying characteristic; it is not a grade.
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