Fluorescence is a greatly misunderstood concept. Fluorescence is the reaction of trace minerals within the diamond that cause the diamond to glow when exposed to ultraviolet light. In the laboratory, special ultraviolet lamps are used to check for this but the sun is also a source of ultraviolet rays so diamonds will react in daylight as well. About one third of all diamonds fluoresce. Of these, the most common color is blue, but diamonds can fluoresce other colors. More than 95% of those that do fluoresce will fluoresce blue. The next most common color is yellow. Any other color of fluorescence would be rare.
There are some educational sites or selling sites that discuss fluorescence as a negative value factor. When a diamond fluoresces blue it has a tendency to appear higher in color than its true body color. This is a good thing. However, many years ago, fluorescence was thought to be a negative, and the reasons can only be speculative. One such reason was that it was thought that perhaps the color grade assigned was lower than its apparent color, so people were fearful that they would be paying too much for a diamond. For example, a J color diamond might look like an H or an I color if it had fluorescence. But as long as the diamond is correctly graded as J, then the fluorescence is really like a bonus. The diamond looks higher in color than the price suggests.
Two factors should be considered regarding fluorescence. The first is the color of the fluorescence. If the diamond fluoresces blue, it may be a positive factor since it will make the diamond look whiter. However, note that there is still a stigma against fluorescence and some people will simply not buy the diamond and some sellers will offer a discount for fluorescent diamonds in the higher colors. If the diamond fluoresces yellow, this is a negative factor because the diamond will look lower in color in ultraviolet light.
The second factor is the strength of fluorescence. The range of strengths as reported on laboratory grading reports is None, Faint, Medium, Strong, Very strong. Some labs use the term Negligible for any diamond with no fluorescence or faint fluorescence. Sometimes when a diamond has very strong fluorescence, the diamond will have an “oily” look to it, even in normal lighting conditions. When this happens, a negative value is expected. The range of discounts that might be realized are anywhere from 0% to 15%, but a large discount is rare. Occasionally, a slight premium of 1% to 3% might be added for a diamond that is in the lower color grades but exhibits fluorescence.