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.