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The tribulations of light

JOHN CHAPMAN explores how light impacts our perception of a diamond and the common mistakes and misconceptions jewellers may unwittingly harbour.

Light – I have always liked it. It transmits, reflects, refracts, diffracts, and gives gemstones their dazzling life.

The absorption of light across visible wavelengths gives a gemstone – and most objects, for that matter – colour. Absorption of green light renders a diamond pink, while absorption of blue makes them yellow.

Since most ‘white’ diamonds have a slight yellow tinge, then injecting a bit of blue into them should make them more white looking, hence the blue tissue inside common brifkas – the envelopes or packets used to hold diamonds.

Jewellers also rely on light to show off their product, but most of them do it badly – though if it is any consolation, museums do it worse!

The lighting ‘experts’ come along, decide darkness is trendy and assume that the stones will stand out like beacons.

I’ve seen walls and ceilings painted black. The lighting technician points a few spotlights at the jewellery and thinks, “Job done”. But look at any diamond in such surroundings and they look like polished coal!

It is such a pity when a customer can’t see the difference between a D and M-colour stone in a cabinet, and a tragedy when visitors to the Smithsonian see the Hope Diamond not as a vivid blue marvel that it is, but more as faceted black glass with periodic flashes of blue as it rotates.

I should know, I’ve seen it in person – and indeed, overheard a saddened visitor say, “It’s very disappointing,” as I tried to explain the problem to a curator.

Diamonds need to be thought of as a collection of mirrors; if the surroundings are dark, a mirror will look dark.

Besides absorption, diamonds can emit light – commonly called fluorescence – which I think is their most maligned property.

When stuck for conversation, I often ask jewellers, “What do you think of fluorescence?” and all too often they reply, “I always buy ‘none’ fluorescence,” with mutterings of “oily look”.

That provides me with an opportunity to launch a discussion about the reality of diamond fluorescence.

Diamonds need to be thought of as a collection of mirrors; if the surroundings are dark, a mirror will look dark."

Having helped HRD Antwerp – one of the world’s leading authorities on diamond grading and education – conduct an extensive consumer study on the topic a few years ago, I have learnt a thing or two about ‘the glow’!

Several decades ago, fluorescence was actually considered desirable and stones exhibiting the phenomenon attracted a premium. Sometime between now and then such stones became persona non grata and were discounted heavily.

“This stone would be worth $60,000 more if it wasn’t graded strong fluorescence,” lamented one diamond dealer in Hong Kong, showing me a 10-carat F-colour.

Rumour has it that fluorescence got a bad name when a new mine came on-stream that was outside De Beers’ control; it just so happened that the diamonds from that deposit tended to be quite fluorescent.

So, to dissuade dealers from buying the renegade mine’s stones, consumers were told that fluorescence was a bad quality and the label stuck. That’s just one of the legends, but I’m open to other theories!

If a diamond’s fluorescence is graded as ‘none’ then most people reasonably expect to see black when observing it under ultraviolet light, but invariably there is a glow. Almost all diamonds fluoresce, it is just a matter of degree; the glow must be below a certain master reference to be graded ‘none’.

The difficulty laboratories had – and probably still have – is that their customers, in their quest for ‘none’, grumble when they receive a grading of ‘faint’.

So, to keep the loyal customers happy, I reckon the ‘none’ threshold has crept up over the years. My theory is supported by the evidence that in 1997 the proportion
of diamonds graded ‘none’ by GIA was 65 per cent and analysing the recent offerings online, I note the figure is now 80 per cent.

I’m sure the fluorescence profile of mines hasn’t changed that much over time!

Going back to the HRD Antwerp study, numerous people were shown diamonds of D-J colour having different fluorescent intensities. It was found that the apparent colour in daylight could be elevated by several colour grades thanks to the fluorescence; a strongly fluorescent J colour could look D!

So, there you have it – nowadays you can buy a diamond discounted for its fluorescence and then it looks like a stone of better colour and hence of higher price.

In some extreme instances a diamond can look milky or oily, but a GIA study found that only 0.2 per cent of fluorescing diamonds fell into that category.

And one more thing – for the fluorescence to manifest itself, ultraviolet light needs
to strike it. Artificial lighting is free of ultraviolet – isn’t it indoors when diamonds are almost exclusively seen and admired?

 

Name: John Chapman
Business: Delta Diamond Laboratory/Gemetrix
Position: Director
Location: Perth, WA
Years in the industry: 35

 

 

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