The 4 C’s are part of a well-known grading system for evaluating diamonds by grading the carat weight, color, clarity and cut. RAPLAB® grades diamonds based on the 4 C’s and several additional criteria using sophisticated technology to provide you with a more detailed and informative certification.

One diamond out of three responds with fluorescence glow if exposed to ultraviolet (UV) rays. The strength, intensity and color of the fluorescence are different for each stone. Just 10 percent of all stones have a steady, strong fluorescence.

Usually, the fluorescence color is blue, though occasionally it is yellow or white. Rare fluorescence colors, like: orange, purple and red. can be found in a very unusual stones.

RAPLAB® gemologists grade fluorescence in order to evaluate its glowing strength and intensity and describe its color, if it has any.


Keep in mind that fluorescence does not affect the stone’s beauty, clarity or structure. Research has not found any connection between the presence of fluorescence and a weak diamond. A few diamonds with enormously strong fluorescence may have a milky or oily appearance.

This is a unique attribute of some diamonds that causes them to “glow” a typically bluish color when exposed to natural or man-made ultraviolet light (like a Black Light). About 50% of all diamonds fluoresce and of those about 10% fluoresce strongly. Most of the time, fluorescence is not a factor unless the intensity is Strong or Very Strong. In the very rare colors D, E, and F, Strong fluorescence stones are consider less desirable. However, in the less rare colors of such as J – K and below, Strong fluorescence may be desirable.


This is a false and misleading term. According to the FTC a diamond must be blue/bluish or colorless. Fluorescence is also a factor. AGS prohibits use of the term.