A weighty problem

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.