Close enough?

May 1, 2013

By Mark T. Cartwright

The 'belly' on cabochon-cut stones can increase the carat weight significantly and can be tricky to judge properly without experience and reference stones.[1]
The ‘belly’ on cabochon-cut stones can increase the carat weight significantly and can be tricky to judge properly without experience and reference stones.

As a professional appraiser, I pride myself on accuracy, clarity, and a defensible methodology. I believe what I’m doing and how I’m doing it is ‘right.’ Recent experiences have somewhat shaken my belief system, and I thought I’d share my concerns with the hope others finding themselves in any similar situations might benefit from my learning curve.

Each of us utilizes a methodology and a plan of action when performing an appraisal. Generally, that’s what is referred to by the term ‘scope of work,’ and it can vary from assignment to assignment. Although each of us is idiosyncratic in the specifics of our approaches, they can always be broken into the broad categories of identifying the problem to be solved, collecting data, analyzing it, drawing a conclusion, and reporting the results. The first category is all about identifying the intended use and the appropriate type of value. The last two categories involve the appraisal narrative and report format. My recent ‘issues’ have involved the second and third steps in the process, the ones few of us ever question. Let’s look at ‘collecting and analyzing the data’ for a moment.

One of the ‘truths’ I have taken to be fairly constant is the relationship between the diameters of melee diamonds and their weight. For instance, for most of the 30-plus years I’ve been appraising, I could reasonably assume that a .01-carat diamond would measure 1.5 mm in diameter; a .05-carat diamond would be 2.4 mm, etc. Lately, it’s become apparent to me the veracity of that relationship may depend on the age of the jewellery and possibly the diamonds’ country of origin. One example is the current fashion for so-called ‘micro-pavé’ and the utilization of new manufacturing techniques that has led to many fairly significant changes in a variety of different areas of our trade.

Getting in closer

So-called micro-pavé is often the result of in situ casting techniques in conjunction with CAD/CAM manufacturing. It can be an appraiser's nightmare.[2]
So-called micro-pavé is often the result of in situ casting techniques in conjunction with CAD/CAM manufacturing. It can be an appraiser’s nightmare.

Concerning the name, micro-pavé is typically not pavé. It is a style featuring many tiny full-cut diamonds that most often have been cast in situ. In my experience, it isn’t unusual for a single .50-carat total weight ring to contain 80 or more diamonds, none of which is larger than 1.3 mm (thus the term ‘micro’). Jewellery designers, especially those using computer-aided design (CAD) techniques, are creating jewellery with diamonds mounted virtually everywhere there is a surface larger than 1 mm. The use of computer-aided manufacturing (CAM) allows a wax model to be created with the settings already fully formed and finished. Beads, or more correctly pinpoint prongs, are pre-drilled and precisely placed in the wax model. This has led to the extensive use of the technique of casting the diamonds in situ. For the manufacturer, the result is a significant savings in both money and time. For the appraiser, it’s a nightmare.

Since the diamonds were set into the wax model prior to the casting process, the pinpoint prongs or beads are often bulky and cover a large percentage of the stone. It becomes impossible to properly finish the metal surfaces around and under the stones, so the contrast between metal and diamond is diminished. For the appraiser, the task of accurately measuring large numbers of melee diamonds to within 0.05 mm can be an opportunity to cultivate patience and focus. When we add to the challenge that they are buried in the metal and mounted at odd angles in tiny spaces, it can be a particularly daunting ‘opportunity.’ It isn’t unusual that somewhere around the 80th stone I ask myself, “Is it really important to measure all these diamonds?” We’ll explore that question more deeply a bit later, so for now I’m prepared to offer a definite ‘maybe.’ But how?! I use a table gauge and sometimes a spot of ink from a ‘gel’ pen to mark my place as I systematically work through the item’s various surfaces. It’s a slow, tedious, and inherently imprecise method, but with care and patience, it can still produce credible results. Maybe someone reading this column has figured out a better way; if so, please share it with us before we all go bonkers!

Lest someone think I’m picking on micro-pavé pieces, everything I’m discussing applies to any style of jewellery comprising many small stones intended to amount to a specific total carat weight. True pavé jewellery, as well as ‘line’ or tennis bracelets, riviere necklaces, and cluster settings, all have the same potential to cause problems for appraisers.

A weighty problem

The irregular outline and shallow make of these old-European brilliant diamonds become apparent at higher magnification.[3]
The irregular outline and shallow make of these old-European brilliant diamonds become apparent at higher magnification.

As the late-night TV salesmen like to remind us, “But wait, there’s more!” As I’ve recently discovered, failing to properly assess the cutting proportions of diamonds as small as 0.6 mm can have a significant impact on a report’s conclusions. Good grief!! Dozens of nearly microscopic diamonds buried beneath reflective prongs in inaccessible nooks and crannies—and now I’m supposed to evaluate the proportions! Sadly, yes. We needn’t address every dimension, just total depth and girdle thickness.

I own a variety of diameter-to-weight conversion charts for diamonds of various shapes that I refer to constantly. I’ve collected them over the years and receive new ones fairly regularly from diamond and gemstone melee suppliers. Perhaps the most striking thing about these charts is how different the more recent ones are from the ones that have been around for many years. Even the current charts vary from each other fairly significantly and the more ‘honest’ of those provide a range of values for each weight and millimeter size. Fancy shapes such as baguette, princess, etc., have the widest variation, but even the round brilliant charts show differences that can add up quickly—and that’s the key.

How often do any of us consider the ‘margin of error’ we must stay within in order to arrive at an accurate estimation of a piece of jewellery’s total diamond weight? Obviously, we always do the ‘best we can,’ but that’s not the same thing as understanding the potential margin of error. As an example, if I’m ‘off’ by 15 per cent on each of 10 stones weighing .01 carats, it’s unlikely the .015 carat total is going to significantly affect value. On the other hand, if I’m ‘off’ by the same percentage on each of 200 stones averaging .05 carats apiece, I could have a real problem accounting for the 1.50-carat difference. That’s why it’s important to consider the melee’s proportions. As a means of checking myself and by using standard formulae, I’ve performed random calculations of the weight of melee in bracelets, necklaces, and rings. In extreme cases, I’ve discovered stones with calculated weights more than 60 per cent greater than their diameters would indicate; 15 per cent heavier was not uncommon. Don’t despair; you don’t have to calculate the weight for all 200 stones—we can use our skills as a gemmologist to provide answers.

Eyeing the problem

The exceptional cutting quality of the aquamarine and diamonds in this bracelet made adjustments for proportion issues unnecessary.[4]
The exceptional cutting quality of the aquamarine and diamonds in this bracelet made adjustments for proportion issues unnecessary.

The easiest way I’ve found to come up with a reasonably accurate and defensible weight correction for small stones is based on the sight estimation techniques many of us were taught in our GG studies. A quick scan down the sides of the rows of diamonds may reveal the modal girdle thickness, as well as the extremes. That could suggest the need to add two to 10 per cent to the total weight derived from diameter measurements. Looking for the table reflections in the diamonds’ pavilions can provide a reasonable idea of the modal pavilion depth percentage. If the majority of the stones have a ‘nail-head’ type of reflection, it may be necessary to add an additional two to five per cent to the total weight. If the stones’ crowns look like ice cream cones, you may need to add yet another two to four per cent. On the other hand, if the diamonds are old-European, old-mine brilliants, or show ‘fish-eye’ reflections of their tables, you may need to reduce your total weight estimate appropriately. The crucial point is to understand that consistent variations from ‘typical’ proportions are common in contemporary and in period/antique jewellery, and those variations must be accounted for when estimating the piece’s total weight.

If you are like me and have been using the same outdated charts and making the same comfortable assumptions about diamond cutting, then you and I have probably been making the same mistakes in total weight estimation. The errors individually may be insignificant. However, when there are large numbers of stones in a piece, the small error is multiplied dozens, or even hundreds, of times and can become significant. The concept of ‘due diligence’ implies, among other things, that one recognizes and corrects mistakes when they become known. We aren’t expected to be infallible—only to exercise due diligence in our effort.

Mark Cartwright[5]Mark T. Cartwright, ISA CAPP, ICGA, CSM-NAJA, GG (GIA) is president of The Gem Lab, I.C.G.A., an independent American Gem Society (AGS)-accredited gem laboratory. He has been a jewellery designer, goldsmith, gemmologist, and appraiser for more than a quarter century. Cartwright can be contacted via e-mail at gemlab@cox-internet.com[6].

 

Endnotes:
  1. [Image]: http://www.jewellerybusiness.com/wp-content/uploads/2013/05/sapphire-cab-necklace-closeup.jpg
  2. [Image]: http://www.jewellerybusiness.com/wp-content/uploads/2013/05/micropave-wed-set.jpg
  3. [Image]: http://www.jewellerybusiness.com/wp-content/uploads/2013/05/OEC-pave-field-closeup.jpg
  4. [Image]: http://www.jewellerybusiness.com/wp-content/uploads/2013/05/aqua-bracelet.jpg
  5. [Image]: http://www.jewellerybusiness.com/wp-content/uploads/2014/12/Mark-Cartwright.jpg
  6. gemlab@cox-internet.com: mailto:gemlab@cox-internet.com

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