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You may have noticed on some diamond certificates a reference to fluorescence. This article explains what diamond fluorescence is, what causes it, how it is graded, and how it affects the appearance and value of diamonds. It also touches on a related phenomenon known as phosphorescence and explains the difference between diamond fluorescence and phosphorescence.
What is diamond fluorescence?
Between one in three and one in four diamonds exhibit a natural phenomenon whereby they glow under ultraviolet light. It is important to note that this fluorescence lasts only for as long as the diamond is exposed to UV light – these diamond stop glowing the moment the UV light source is removed.
More than 95% of diamonds that react to UV light in this way glow blue. Most of the remaining 5% glow yellow, though a few have a green glow, and in extremely rare cases, they glow red.
Blue and red fluorescence are caused by trace amounts of nitrogen within the diamond, while trace amounts of boron and aluminium cause the other fluorescence colours.
Diamond fluorescence grading scale
Gemologists can assess the degree of fluorescence that a diamond exhibits, and the GIA, for example, grades diamonds from least to most fluorescent on the following scale:
- None
- Faint
- Medium
- Strong
- Very Strong
Does fluorescence make a diamond appear hazy?
A study by the GIA found that fluorescence does not affect the transparency of diamonds except in two very rare cases: diamonds with yellow fluorescence and those with very strong blue fluorescence (known in the trade as "overblues"). These unusual stones can appear hazy, cloudy, milky or oily when viewed in bright sunlight.
Does diamond fluorescence affect the value of a diamond?
Generally speaking, diamonds that exhibit fluorescence trade at a discount to diamonds that do not since there is decreased demand for them, with most consumers preferring to choose diamonds without fluorescence.
As you might expect, the size of the discount increases with the intensity of the fluorescence, such that the discount for faint fluorescence tends to be relatively small, but for strong fluorescence, the discount can be very significant.
However, it should be noted that faint or even medium blue fluorescence can enhance the appearance of a diamond. Blue is the complementary colour to yellow, and since it is a yellow hue that is present in most white diamonds (other than those graded D colour), some blue fluorescence can "cancel out" the yellow and make the diamond appear whiter than it is.
What is phosphorescence in diamonds?
Rarer than fluorescence, phosphorescence refers to the property of some diamonds to glow after exposure to a high-energy light source and to continue to glow even after that light source has been removed. This glow can last for a few seconds to several minutes, depending on the type and intensity of the original light source.
Very rare in nature, some lab grown diamonds grown by the high pressure, high temperature (HPHT) method also exhibit phosphorescence, emitting an orange glow after exposure to intense light.
Like fluorescence, diamond phosphorescence is caused by UV light interacting with trace elements such as nitrogen and boron within the diamonds. However, unlike fluorescence, it is not disclosed on gemological diamond reports or certificates, so if you want to be sure that you don't purchase a diamond that exhibits any phosphorescence, you need your jeweller to check and confirm this before placing your order.
Summary
Diamond fluorescence is a natural property found in between a quarter and a third of mined diamonds. Typically, it causes a diamond to glow blue under UV light, though other colours are also possible. It usually has no negative impact on the appearance of a diamond in daylight, except in the rare cases of yellow and very strong blue fluorescence. These unusual stones can appear hazy in normal conditions.
Fluorescence is graded from none, through faint, medium, and strong to very strong. Usually, diamonds with fluorescence trade at a discount, which increases with the degree of fluorescence. This is even though faint or medium blue fluorescence can make diamonds look whiter by reducing their yellow hue.
Diamond fluorescence should not be confused with phosphorescence, whereby a diamond continues to glow even after it has been removed from a light source. This rare phenomenon is present in a very small number of natural diamonds and some lab grown diamonds. Unlike fluorescence, it is not disclosed on gemological diamond reports or certificates.
