Fluorescence is not directly related to a diamond's color. This separate characteristic refers to the diamond's ability to fluoresce under ultraviolet (UV) light. Our sun emits some UV light, but it is usually not great enough to detect fluorescence. When exposed to UV light, many diamonds will give off a distinctive glowing blue coloration. Although fluorescence may be displayed in various colors, blue is the most common in diamonds. The fluorescence of a diamond is defined by its intensity as either None, Faint, Medium, Strong, or Very Strong. Although fluorescence is a characteristic that can be measured, it is rarely an issue when selecting a diamond and is usually not detectable to the eye.
The impact of fluorescence on price depends on its noticeability. Faint fluorescence has very little effect on a diamond of any color, and therefore has no effect on value. For some higher color stones (D-G), strong fluorescence may give the stone a milky white appearance, which greatly lowers value. Fluorescence often adds value to lower color stones, such as I and lower, as it gives the stones a whiter, brighter appearance.