Fluorescence (a variety of luminescence) is the light radiation due to a reaction of the material atoms when exposed to incident light. In diamonds, we can check the presence of fluorescence with long-wave ultraviolet light. This glow-like effect most commonly appears blue under UV illumination. Ultraviolet light is present in a variety of light sources, including artificial (office) light and sunlight. Incandescent light is very low in UV content, whereas sunlight and fluorescent- fixture light are much stronger in UV. Blue is by far the most common color of fluorescence, but green, orange, white, yellow, and combinations thereof, can also occur.