The Essential 7C's
Diamonds that are said to be fluorescent contain particles that emit a visible (usually blue) glow
when exposed to UV lighting. In rare cases, high levels of fluorescence can make stones appear milky
or hazy, although for the most part fluorescence does not generally impact beauty or sparkle and can
even make some lower colours (I, J, K, L, etc.) appear more colourless or white. Nonetheless,
fluorescent diamonds are usually cheaper and provided the effect is not readily visible may offer
good value for money.
The subject of diamond fluorescence has been hotly debated in recent years, with trade opinion divided
regarding its effect on the appearance and value of diamonds.
Fluorescence occurs in some diamonds when they are exposed to the concentrated radiation of a UV lamp.
Submicroscopic structures in the diamonds cause them to emit a visible light, a fluorescence, which is commonly blue
in colour. Despite the fact that fluorescence is used as an identifying characteristic rather than a grading factor
in most laboratory quality reports, its presence in such reports is being used with increasing frequency to determine
the pricing of a diamond. In most cases, prices are lowered when a colourless or near-colourless diamond fluoresces
under UV light, due to a common perception that fluorescence has a negative effect on the appearance of diamonds.
However, there are many who believe that fluorescence has no adverse effect; some even contend that it enhances
The negative image of fluorescing diamonds can be attributed to the following theories and factors:
- Non-fluorescent diamonds are regarded as ‘purer’;
- Strongly fluorescent diamonds can emit an opaque ‘cloud’ which affects the clarity and brilliance of the diamond;
- GIA lab grading lights emit a small amount of UV radiation, therefore fluorescent diamonds might be assigned a better colour grade than they deserve.
Those who refute the notion that fluorescent diamonds are less desirable argue that:
- UV light is not present in many viewing environments, so it makes sense to grade colour in realistic lighting;
- The haziness found in highly fluorescent diamonds is extremely rare;
- In some cases, fluorescence can lift colour to the naked eye, enhancing the beauty of the diamond.
A 1997 study conducted by the GIA concluded that, in most cases, fluorescence does not impact the face up
appearance of a diamond. In fact, they found that “strongly blue fluorescent diamonds were perceived to have a
better colour appearance when viewed table-up” compared to non-fluorescent diamonds. (Source: A Contribution To
Understanding The Effect Of Blue Fluorescence On The Appearance Of Diamonds, by Thomas M. Moses, Ilene M. Reinitz,
Mary L. Johnson, John M. King, and James E. Shigley)
In Europe, before the age of diamond certification, the most highly regarded diamond colour was
‘blue white’. This term describes near-colourless to light yellow diamonds with a strong blue
fluorescence, stones that were actively sought by merchants thanks to their appealing ‘ice’ effect.
It was later observed that strong blue fluorescence was a quality sometimes found in hazy stones,
a fact that that led dealers in the 1970s to offer what they termed ‘milky Ds’ (diamonds with a colour
grade of D, very strong blue fluorescence, low transparency) at significantly reduced prices. This had
a knock-on effect during the years that followed: eventually even F grade diamonds and those with a
weaker fluorescence were being marked down in price.
WHY DO DIAMONDS FLUORESCE?
Diamond is crystallized carbon. This process occurs about 75-125 miles below the surface of the
earth, usually where volcanic or ancient volcanic sites exist. A diamond will become fluorescent
if there are traces of the mineral boron present in the earth during the crystallization process.
Diamonds can fluoresce in a variety of colours – blue, yellow, green, orange and white – but blue is
the most common. It is estimated that around 50% of all diamonds have fluorescence that can be seen in
special conditions e.g. under a short wave UV lamp, while around 10% fluoresce strongly enough to make a
noticeable difference to the colour of the diamond when viewed in sunlight or incandescent (low UV) light.
There are five levels of diamond fluorescence described by the GIA: None (i.e. no fluorescence),
faint, medium, strong, very strong. According to the GIA Gem Trade Laboratory, strength of fluorescence
does not directly correlate to either colour or clarity. In other words, it is possible for two diamonds with
completely different clarity and colour grades to exhibit exactly the same strength of fluorescence.
Figure A & Figure B show the varying levels of fluorescence on diamonds viewed table-down and table-up. The levels are as follows: 1. Very strong, 2. Strong, 3. Medium, 4. Faint, 5. None.
Figure C shows the relationship between absorption and luminescence (the emission of light by a substance that has
absorbed UV radiation.) The figure shows an electron moving from the ground state to an excited state
(north arrow) when UV radiation is absorbed at a luminescence center. When the electron moves to a slightly
lower state it loses some of its energy (wavy arrow) and no light is emitted. Subsequently, it returns to the
groundstate (straight down arrow), releasing energy, or light, as fluorescence.
HOW DOES FLUORESCENCE IMPACT DIAMOND PRICING?
Historically, strongly fluorescent diamonds have been marked down in price next to their non-fluorescent
counterparts. Typically, higher colour (D-H) non-fluorescent diamonds will sell for 10-30% more than similar
stones that exhibited very strong blue fluorescence. However, lower colour (I–N) diamonds with very strong
fluorescence were priced up to 5% higher than similar non-fluorescing stones. This is likely due to the
perception that blue fluorescence can mask the undesirable yellow tones found in some stones.
THE GIA STUDY
Despite a long history of debate on the issue of fluorescence and pricing, there had been no studies that
examined the influence of blue fluorescence on the appearance of a diamond under normal viewing conditions
until the GIA undertook their 1997 experiment.
Colour assessment in grading laboratories takes place under carefully controlled lighting and viewing
conditions and mainly with the diamond positioned table-down. However, in a jewellery store or showroom a
diamond will normally be examined table-up and in a variety of lighting conditions, as it almost certainly
would be when the item is worn. With this in mind, the GIA set up their experiment to study the influence of
fluorescence in a range of lighting conditions and by observers from both within and outside the diamond industry.
In the experiment, a range of diamonds with very similar proportions, clarity and symmetry, but differing ranges
of blue fluorescence, were assembled. Diamonds of similar colour grades were grouped into sets to represent
important commercial breakpoints. These sets were then examined in a wide variety of viewing environments,
from the controlled conditions found in laboratories to rooms with only natural sunlight. A total of 46 observers
were assembled to represent both the diamond industry and the diamond-buying public.
For the non-industry observers, considered to represent the diamond consumer, fluorescence had no effect on
colour and appearance. For the experienced industry observers, the GIA found that the strength of fluorescence
had no widely perceptible effect on the colour appearance of diamonds when viewed table-down. What’s more, when
the diamonds were viewed table-up, as they would be in a jewellery store or showroom, the experienced observers
actually reported strongly and very fluorescent diamonds as having a better colour appearance than the less
The GIA discovered that while the effect of fluorescence on colour was most noticeable in stones within lower
colour grades (I and K), in the marketplace the influence on price is greater in stones with higher colour grades
(D through H). This challenges the trade perception that fluorescence usually has a negative effect on
better-colour diamonds. They also found that, contrary to popular belief, fluorescent diamonds aren’t as common
as non-fluorescent diamonds.
The GIA report concluded that "the diamond industry would be better served by considering each individual diamond on its own visual merits."
SHOULD I BUY A FLUORESCENT DIAMOND?
It’s important for you to be happy with the diamond you purchase. As shown, fluorescence can greatly enhance
the colour and appearance of a diamond and shouldn’t be looked upon unfavourably; only in exceptionally rare
cases will fluorescence cause the diamond to look milky, oily or hazy. It always comes down to a subjective
decision, but best practice would be to consult a diamond specialist before purchasing to ensure you make the