Other than the 4 C’s : Fluorescence
The 4 C’s are important, but gemologists also take other factors into consideration. Flourescence is one of those factors.
Some gem-quality diamonds fluoresce: they emit light when exposed to long-wave ultraviolet light. Fine quality diamonds with strong fluorescence, called “overblues” have a visible haziness that makes them appear almost cloudy in light with strong ultraviolet content. At the same time, strongly fluorescent diamonds with a yellowish body color have long been considered to appear to have a better color because the blue of the fluorescence makes them appear more white in sunlight, which is a source of ultraviolet light.
Lower-color diamonds with strong fluorescence sometimes command a premium. The Gemological Institute of America lists fluorescence as an identifying characteristic, not a grading factor. And based on a random sample of 26,000 diamonds, fluorescence is more rare than nonfluorescent: 65 percent of diamonds do not fluoresce.
If you are ever in the Smithsonian Institution in Washington D.C., you can see the most famous example of an overblue: the 127 carat Portuguese Diamond. While you are there, make sure to visit the famous Hope Diamond, which owes some of its legendary curse to the fact that it fluoresces an extremely unusual red, which is only known to happen in blue diamonds.
More Grading Considerations:
Symmetry is an important element of a quality finished diamond. Symmetry means the exactness of the shape and the balanced arrangement of the facets.
Polish influences how well light is able to pass through a diamond and is important to a diamond’s brilliance. Diamonds that have poor to extremely poor polish are less brilliant because they have microscopic polish lines that blur the surface of the diamond. These polish lines reduce the amount of light that enters or exits a diamond.
The depth of a diamond is important to its brilliance and value. Diamond cutters must remove more weight from the original rough diamond crystal to cut a diamond with proportions that produce great brilliance.