Diamond Phosphorescence

Most diamond shoppers who have spent any time at all looking at diamonds, or laboratory reports on diamonds, have encountered a trait called fluorescence. This characteristic has long been a part of diamond evaluation as it helps to positively identify a diamond and because in some cases fluorescence can impact both a diamond’s light performance and its value. A related characteristic called phosphorescence is by far lesser known and is typically not contained on a diamond report.

In this article we will discuss phosphorescence and its relationship to fluorescence, and its impact on diamond quality and price. The new surge in lab grown diamonds has brought this trait more mainstream as lab diamonds appear to be more likely to display phosphorescence. Interestingly, fluorescence is quite rare in lab grown diamonds.

Fluorescence is a reaction to UV light found in about 30% of natural diamonds. Defects in the carbon lattice, typically associated with traces of nitrogen, cause these diamonds to emit light and glow as long as the UV light source is present. Most diamonds are inert and do not react to UV light in this way. They are reported to have ‘none’ on a diamond certificate for fluorescence. But a significant number of natural diamonds have observed fluorescence from faint to very strong, and the color is usually blue.

The energy imparted by the UV light source causes electrons associated with the impurities in the carbon lattice to jump to a higher orbit. As they fall back to stable positions they give back the energy gain in the form of light. In fluorescence the emission of light, the blue glow, stops as soon as the stimulating light source is extinguished.

Diamond fluorescence has been a hotly debated topic within the natural diamond trade over many years. Strong blue fluorescence can have a whitening effect on diamonds under certain circumstances, making a diamond appear more colorless than its color grade would indicate. It can also have some negative effects for transparency or contrast. For more information see our page on the pros and cons of diamond fluorescence.

Diamond phosphorescence is a similar trait to fluorescence resulting from different trace elements and/or defects in the carbon lattice. The chief visual difference is that the glow emitted by a phosphorescent diamond persists for a period of time after the light source is turned off. Phosphorescence can last from seconds to minutes. The effect can also be faint to very strong and is commonly seen in colors other than blue. Strong red phosphorescence is a notable characteristic of the Hope Diamond, one of the most iconic diamonds in the world.

Natural fancy blue diamonds like the Hope get their unique color from the trace element boron. There is a strong relationship between the presence of boron and phosphorescence. As boron is an element that is commonly present in the production of lab-created diamonds, it is therefore not surprising to see the interest in phosphorescence increasing as more attention is paid to this emerging market.

Fluorescence and phosphorescence are studied in the laboratory using sophisticated luminescence spectroscopy to measure the wavelengths of light that stimulate these effects and the wavelengths of light that are emitted by the diamond during exposure to stimulation, and after the light source is extinguish (in the case of phosphorescence). Some diamonds both fluoresce and phosphoresce, often at different strengths and sometimes even different colors.

The main difference from a visual standpoint is that a fluorescent diamond will emit light only while being stimulated by a sufficiently intense UV light source. A phosphorescent diamond will continue to emit light for a period of time after the light source is removed.

A fluorescent diamond will glow under a black light; a phosphorescent diamond will glow in the dark! Think of a burning ember that is taken out of the fire – it will continue to glow for a while but will gradually fade away.

If your diamond glows in a dark room with no light source, you have a phosphorescent diamond. If the effect is very noticeable, the diamond is likely to be lab grown. If the diamond is natural and has obvious phosphorescence that persists over a significant amount of time (minutes), you have a very rare diamond.

The visual effect of diamond phosphorescence is subtle and rarely noticed. You really have to go looking for it in order to even detect it in most cases. In natural diamonds the effect tends to be weaker and of shorter duration. It is being noticed to a greater extent today because of the prevalence of lab grown diamonds in which phosphorescence tends to be stronger and more noticeable.

Fluorescence is observed in the laboratory under special, standardized UV lighting. But it can also be seen in some real world circumstances, most vividly in dark rooms with black lights such as in some night clubs. A strongly fluorescent diamond will glow brightly. Sometimes such an occasion is the first time a person becomes aware that they have a fluorescent diamond, and it can come as a big surprise!

The atomic level defects that cause phosphorescence are often distinct from those that cause fluorescence and commonly result in the emission of orange light. Also different is that, in order to detect phosphorescence, the diamond must be exposed to the proper light source to stimulate the effect, then you must immediately turn out all lights to look for the telltale glow. The color is often orange (especially in lab grown diamonds), but can be other colors as well. There have been documented cases of diamonds having two different colors of phosphorescence of different durations, such that the observed color changes over the duration of the effect!

Above: A series of images showing the gradual change (left to right) in the phosphorescence of an HPHT-grown colorless diamond after removing it from the LWUV source (left image) in increments of 30 seconds for 150 seconds (right image). Bottom: The gradual change in phosphorescence after exposure to a SWUV source (left image) in increments of 30 seconds. The greenish blue and orange emissions decay differently, with the blue fading faster (~90 seconds, fourth image from left) than the orange (~240 seconds, right image).

If phosphorescence is strong and of long duration, it is an effect that can be easily observed. In such a case, it is more likely than not to be a lab grown diamond as the effect tends to be much weaker in natural diamonds.

It normally lasts from seconds to minutes, but can potentially have longer duration. It depends on the trace elements or other subatomic features in the diamond, and on the light source stimulating them. For example, as with fluorescence, the observed phosphorescent reaction can be very different when stimulated with short wave vs long wave energy.

The fluorescence observed on a GIA report is stimulated by Long Wave UV at 365nm. Diamonds can have very different reactions to stimulation by Short Wave UV at 200-240 nm. For instance, most lab grown diamonds do not react to LW but under SW will reveal aspects of their structure that enable conclusive identification as synthetic. Detection devices can use shortwave stimulation to distinguish natural from lab grown diamonds.

Does the intensity of phosphorescence indicate the quality of a diamond? There is no evidence that phosphorescence has any impact on a diamond’s light performance or appearance in normal lighting environments. But because it is a rare phenomenon in natural diamonds there seems to be a stigma against it in the lab grown diamond market, similar to the market’s view of blue nuance, which is also related to boron impurities. To the extent that a lab grown diamond has visual markers that reveal it to be synthetic, it tends to be penalized by the market.

Yes, in fact phosphorescence is more common in lab grown diamonds than in natural diamonds because the element boron is often used in the growth process. Conversely, fluorescence is much less common in lab grown diamonds because nitrogen, the trace element most responsible for diamond fluorescence in natural diamonds, is rarely present in the process used to create colorless lab grown diamonds.

HPHT grown diamonds intended to be colorless often have significant traces of boron, sometimes resulting in a faint blue body color known as ‘blue nuance’. These diamonds also have a high propensity for phosphorescence which can be of higher strength. And like blue nuance (which is arguably an attractive body color for a diamond), phosphorescence tends to seen as a negative in the lab grown market today.

It seems that there is also a relationship between the impurities of nitrogen and boron, such that the presence of nitrogen can suppress phosphorescence in diamond. Most lab-grown diamonds are type IIa which is characterized by unmeasurable amounts of nitrogen. With the presence of significant traces of boron tending to make phosphorescence stronger, the lack of nitrogen which is present in larger amounts in the vast majority of natural diamonds, leaves lab grown diamonds displaying phosphorescence to a far greater degree than naturals, particularly those grown by the HPHT method.

Phosphorescence is an interesting trait seen in some natural diamonds and a great many lab grown diamonds. After being stimulated by a light source for a period of time a phosphorescent diamond will continue to glow after all light has been extinguished, gradually fading in a few seconds to a few minutes. This effect differs from fluorescence in that the fluorescent effect is only exhibited as long as the stimulating light source is present.

Phosphorescence is strongly associated with the trace element boron which is rarely present in natural diamonds, but is commonly present in the growth environments of lab created diamonds, particularly those grown by the HPHT method. Colors and strengths of phosphorescence can vary as well as their durations. Natural diamonds tend to be weaker and of shorter duration than lab grown diamonds.

Phosphorescence is not known to have any impact on a diamond’s quality or appearance in normal lighting environments. However, there seems to be a stigma against phosphoresce in the lab grown diamond market, similar to blue nuance, tied to the fact that these boron related traits are so rare in natural diamonds.