Fluorescence is far more complex than most people
realize. When a diamond is fluorescent it typically
appears whiter when exposed to sunlight than under
incandescent light. This is because UV light (less than
380 nm in wavelength) contained in natural sunlight
stimulates a fluorescent diamond to typically emit blue
light thus making the diamond appear whiter.
Most fluorescent lamps generate a small amount of UV
in their output which can also make a diamond viewed
under such a lamp appear whiter.
The fluorescent phenomena can be observed by holding
something fluorescent (most white paper for example)
in the Colorimeter UV viewing chamber after pressing
the UV key. The UV chamber injects long wave UV
(365 nm) into the area above the black glass window.
Because most visual diamond grading is done using a
fluorescent lamp the grading results for fluorescent
diamonds can change depending on the characteristics
of the lamp as well as how close the diamond is held to
the lamp.
Both visual grading and Colorimeter grading of
strongly fluorescent stones can give misleading results
and some caution is advised whenever you attempt to
grade a strongly fluorescent stone.
• Fluorescent stones can give misleading results.
Always use the UV viewing chamber to observe
if a diamond is fluorescent. Remember, visual
grading of fluorescent stones is very dependent
on the light source being used.

Do NOT rely on the results!!!!!!! and careful of labs that do.