A glow, usually of a bluish color, which emanates from certain diamonds when exposed to UV / Ultraviolet Light. “Extremely Strong” and “Very Strong” fluorescence should be avoided in most diamonds, but anything else from “Strong” to “Faint” fluorescence usually does not affect the appearance of a diamond negatively.
In fact, “Faint” or “Moderate” blue fluorescence is preferred by some customers and many dealers because it can make a less expensive yellowish color appear more white or colorless in daylight, due to the UV in the natural solar light spectrum.
Buyer’s Hint: Because fluorescence is so misunderstood, people are overly concerned about it, making diamonds like this less in demand, even though they look great. As a result, they are often the best deals.