Eyeing the problem

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If you are like me and have been using the same outdated charts and making the same comfortable assumptions about diamond cutting, then you and I have probably been making the same mistakes in total weight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 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.