When diamonds display a visible glow when exposed to ultraviolet (UV) light this is referred to as its fluorescence. Nearly all diamonds will have some degree of fluorescence. For many lighting conditions, a diamond's fluorescence will not be visible to the naked eye. However, it may release a soft colored glow if held under an ultraviolet lamp. Fluorescence is a naturally occurring phenomenon created from the mineral properties boron and nitrogen. It can appear in many minerals and gems.
Diamond fluorescence occurs in varying intensities and colors. Gemological laboratories rate the fluorescence of each diamond on a scale ranging from "None" to "Very Strong". Fluorescence is categorized as 'None' when there is no visible reaction under ultraviolet light. It will be categorized as 'Faint', 'Medium', 'Strong', or 'Very Strong' depending on how much glow is observed. The color range of the glow varies from standard blue to shades of green, white, pink, yellow, red or orange. See the below chart for more details.