Diamond Fluorescence

There are some diamonds that will glow in the dark under a black (ultraviolet or UV) light. This property is called fluorescence. It is also a commonly misunderstood aspect of diamonds. About 30% of gem-quality diamonds fluoresce. Because sunlight is about 10% UV light, this property can have an impact on the everyday use and wearing of diamonds. Why does this happen? And, what sort of problems might occur?

Diamonds are made of carbon atoms very tightly bonded together. But, down deep in the earth where diamonds are formed, there are other elements or minerals as well. Fluorescence happens when a diamond includes tiny amounts of trace elements such as boron, aluminum, and nitrogen.

The Gemological Institute of America once studied this issue by examining 26,000 randomly-chosen gem-quality diamonds. Two-thirds of those diamonds did not fluoresce under a UV light. Of the third that did, a third had a faint blue fluorescence and two-thirds had blue fluorescence in the medium to very strong range. Other colors in which a diamond may fluoresce include red, yellow, and green or combinations such as orange or blue-green. However, fluorescence in colors other than blue is very, very rare.

In general, diamonds that fluoresce have normal sparkle, brilliance, and fire. And the trace elements that cause fluorescence are present in such low quantities that they have no effect on the hardness of the diamond.

Why is this important? Fluorescent does not just occur under the black light on the disco dance floor. Fluorescence occurs from the ten percent of UV light in direct sunlight. Very rarely, a strong blue fluorescence will cause a diamond to appear milky or oily. A less strongly blue fluorescence can be a problem that diminishes the appeal of a D or F color grade diamond. On the other hand, blue is a complementary color to yellow. Thus, blue fluorescence in a lower grade slightly yellow diamond will help cancel out the yellow.

Fluorescence Diamond Grading

The Gemological Institute of America has developed a grading scale for fluorescence. Before we look at the grading scale, there are a couple of facts to remember.

Fluorescence affects the appearance of a diamond in less than 1% of all gem-quality diamonds. And, we simply do not carry diamonds graded at the very strong fluorescence level.

Diamond Fluorescence Grades

Grade: None

When this diamond is exposed to UV light there is absolutely no glow in blue or any other color. This is a diamond with none of the trace elements that might otherwise cause it to fluoresce. Diamonds in this grade sell at a premium as they are considered more desirable.

Grade: Faint

A diamond at this grade gives off a soft blue glow under UV light. This is caused by trace elements within the diamond’s carbon matrix. This level of fluorescence does not affect clarity or sparkle. And, like the “none” grade does not reduce the value of the diamond.

Grade: Medium

In this grade of fluorescence, UV light causes the diamond to glow a more noticeable blue. This is still not enough to hurt the look of a high color grade like a D or F. But, at this grade of fluorescence, color grades J, K, and L will be improved!

Grade: Strong

A very noticeable and almost deep blue glow is caused by UV light in diamonds of this grade. Fluorescence of this grade is a help to J, K, and L color grade diamonds as the blue helps cancel out the yellow. This grade is a problem for D and E color grade stones as they will start to look hazy or slightly gray.

Grade: Very Strong

In this grade, UV light causes the diamond to glow brightly in blue. This definitely affects the diamond’s look, especially in high clarity grades. The perfect D, E, and F color grades develop a gray haze.

Fluorescence, Sparkle, and the Price of the Diamond

As we noted at the beginning of our discussion about fluorescence, it rarely does any damage to the sparkle, brilliance, and fire of a gem-quality diamond. The rare instances where problems occur are with the strong and very strong grades. And this is only when those grades of fluorescence occur in a high-quality D, E, or F color grade white diamond.

The upside to fluorescence is that when it occurs in a low color grade in the J, K, or L range, the blue fluorescence serves to cancel out the yellow tint of the diamond and makes it more attractive. These lower grades will appear to be about a grade higher if they fluoresce. The higher color grade diamonds will have a ten to fifteen percent price discount if they have high fluorescence.

The bottom line here is that fluorescence should not be a deciding factor unless it is strong or very strong. Otherwise, this is a diamond characteristic that you can ignore.

Diamond Fluorescence: Good, Bad, or Not Important?

The GIA, in their study of fluorescence in diamonds, concluded that this quality has no real effect on sparkle, brilliance, or fire of diamonds. Our years of experience in the diamond business have shown us that in maybe one or two percent of diamonds there will an issue that deserves our attention. We deal with the matter of strong fluorescence in high color grade diamonds by simply not carrying any of these. The milky, hazy, or oily look in a high color grade diamond with strong fluorescence is a rare thing but one issue that we simply choose to avoid by not dealing in these gems.

This is an example of fluorescence helping the look of a diamond. Color grade in white diamonds is highest when there is no color and lower when the diamond starts to show hints of yellow or brown. And, the vast majority of fluorescent diamonds emit a blue color. Since blue and yellow are complementary colors, the blue helps to decrease the yellow in a lower color grade diamond. This not perfect, so a J, K, or L grade does not become a D, E, or F grade. But, a lower color grade diamond will often look one grade better with fluorescence.

The bad example is when someone buys an excellent color grade diamond like a D or F but does not check fluorescence. When these diamonds fluoresce they take the diamond down a grade or worse. And, when fluorescence is very strong in an otherwise very clear diamond, the gem has on oily appearance and will look darker. Again, we simply avoid this rare issue by not carrying such diamonds.

One way in which fluorescence is neutral is that does have an effect on the hardness of a diamond.