About 30% of colorless diamonds react to UV rays by showing some degree of fluorescence.
This “glow” is almost always blue, but in rare cases it can appear yellow or another color. Blue being the complementary color to yellow, a blue fluorescence might be considered positive because it can make a colorless diamond look whiter.
Although fluorescence was quite tolerated until the end of the 1990s, a diamond that fluoresces is now considered less valuable than one that doesn’t.
Laboratories highlight fluorescence using specific UV lamps and grade the strength of fluorescence from “faint” or “negligible” to “very strong”.
Then if fluorescence has been found in a diamond, it is mentioned on the stone’s certificate.