Diamond Fluorescence

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.

Introduction

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 colour appearance.

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)

Historical Background

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 long 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 fluorescent stones.

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 right choice.