When exposed to ultraviolet light some diamonds exhibit a type of light, called fluorescence, which can be measured as inert, light, medium or strong. Although diamonds can display fluorescent shades of white, yellow, orange and other colors, the most common of these are light blue. Under normal lighting conditions, fluorescence usually does not affect the appearance of a diamond at all. A yellow diamond may be whiter in color due to some light blue fluorescence, but in some rare cases this fact can make a stone look milky or oily. This milky or oily effect only concerns a small number of "strong" and "very strong" fluorescent stones and is called "over blue"