Rapaport Magazine
Colored Gemstone

Wizardry Weight Computation

We compare two systems — the tried-and-true and the new — for calculating the carat weight of gemstones.

By Robert G. Genis
RAPAPORT... Someone walks into your jewelry store and wants to sell you a mounted diamond. You are sitting in a bank vault and the heirs ask you to bid on a large emerald ring. The owner of an antiques store shows you a ruby in an old mounting and you are interested in buying. You are an appraiser and a client with numerous mounted pieces wants to know their value. In any of these commonplace scenarios, it would be helpful to be able to quickly and easily estimate the carat weight of a gemstone that is not loose and has no grading report.

The tried-and-true method has always been the hardback book, The Complete Handbook for Gemstone Estimation, by Charles Carmona. Recently, David Marcum, president, Gem Weight Wizard, Cape Coral, Florida, created a new computer program named Gem Weight Wizard. It will be downloadable to your PDA, computer, laptop and some phones. RDR thought it would be interesting to compare both methods.


The Complete Handbook for Gemstone Estimation is jam-packed with tables and charts to help you simply estimate the weights of stones. The tables include 24 common and 48 uncommon shapes. There are also sections for pearls and diamonds. More than 100 different gemstones are divided into eight groups based upon their specific gravity. Gem materials vary greatly in density, which is why two different gemstones may have the same size, but different weights, and vice versa.

If you know the gemstone’s shape, length, width and specific gravity group, you can find its approximate weight range using the book’s tables. Each table is for a specific shape, and gives weight ranges for various sizes of each of the specific gravity groups. All of these measurements are standard calibrated sizes; you may not find the exact millimeter measurements of any individual stone. All the tables are based upon the depth percentage — depth divided by width — of well-cut stones. If a stone is deeper or shallower, a correction is made. Adjustments are also made for high crowns, shallow or deep pavilions and any bulge factors. After making adjustments, you come up with a weight range, except for round diamonds, for which the table gives an exact weight.


The Gem Weight Wizard was tested on an HP iPAQ Pocket PC. Once the program opens, it offers a choice of three types of stone from three categories: diamond, corundum or emerald. A pull-down menu includes 29 other less-common gemstones. Once you have chosen the type of stone to be measured, you select the shape of the gemstone from a menu with the most common shapes. The next step is to measure exactly the gem’s length, width and depth and input the measurements into the program. You can also select an outline for hearts, ovals, pears and marquise shapes. Then you select a girdle thickness from thin to extremely thick. The last component of this program is selecting a pavilion bulge. Click on results and you will see a summary of the previous results and the gem’s probable exact weight.


To ascertain their accuracy, we used both of these tools on a series of diamonds, rubies, sapphires and emeralds with known weights. These are all independently graded stones and calculations that were made based on the dimensions given on the stones’ grading reports.

Diamond Observations

The Gem Weight Wizard was consistently higher than the actual diamond weights. The Complete Handbook for Gemstone Estimation tended to be within the projected ranges.

Ruby and Sapphire Observations

Both systems did fine jobs with these stones. The Gem Weight Wizard was within acceptable tolerances and The Complete Handbook for Gemstone Estimation was usually within the general ranges.

Emerald Observations

The emerald weights calculated using the Gem Weight Wizard were sometimes high and sometimes low. The Complete Handbook for Gemstone Estimation did an excellent job, with the exception of being high on the 5.38 emerald.

General Observations

The Gem Weight Wizard should be more accurate because it calculates its weight estimate based on exact millimeter dimensions. However, in this test, the variables, such as bulge or girdle thickness, had to be adjusted in order to make the estimation closer to the actual weight. One disadvantage of using the book is that the tables strictly use calibrated sizes, and the gemstones used were not cut to calibrated sizes. However, through practice and error, it was found that it was almost always better to round up to the next closest calibrated size. This often resulted in spot-on estimations. The bottom line is you need to practice with either system to get a feel for the adjustment factors. Neither is perfect without some corrections. The more stones you weigh, the better you become at estimating the weight of diamonds and gemstones.


These tools are excellent references for jewelers, gem dealers, estate jewelry dealers, pawnshops or jewelry appraisers. But this is a subjective process and if you need to have a precise weight, the stone should be removed and weighed. Each of these tools, however, is better than simply “eyeballing” the stone and guessing the weights of diamonds and gemstones.

According to Marcum, the Gem Weight Wizard used in this test was “one of five beta testers. We will be refining the product based upon feedback and expect to have the finished program in 60 days.” The expected retail is $99 and it will be available at gemweightwizard.com. Your other option is to purchase The Complete Handbook for Gemstone Estimation for $60 and keep it with you in situations where you might need it. Since both systems are reasonably accurate within tolerances, possibly the decision should be based upon your computer literacy or preference. If you like toys and gadgets, buy the Gem Weight Wizard. If not, buy The Complete Handbook for Gemstone Estimation at any major bookstore or online bookseller.

Gem Shape Oval Pear E/C Round Round
Actual Weight 1.03 2.03 3.05 5.46 13.04
Weight Wizard 1.11 2.16 3.33 5.62 13.78
Gemstone 1.01– 1.93– 3.00– 5.30 13.21
Handbook 1.10 2.24 3.23

Gem Shape Cushion Cushion Oval Round Oval
Actual Weight 1.01 1.41 1.53 2.36 3.02
Weight Wizard 0.98 1.36 1.51 2.28 3.12
Gemstone 0.99– 1.38– 1.45– 2.20– 2.92–
Handbook 1.07 1.48 1.56 2.36 3.15

Gem Shape Oval Cushion Cushion Oval Cab Round
Actual Weight 2.09 2.37 3.05 11.16 38.71
Weight Wizard 2.09 2.55 2.94 11.12 37.43
Gemstone 1.85– 2.39– 2.99– 10.50– 34.89–
Handbook 1.99 2.75 3.24 11.37 37.57

Gem Shape E/C E/C Cushion E/C E/C
Actual Weight 2.05 2.10 2.41 3.07 5.38
Weight Wizard 2.21 2.06 2.13 3.41 5.84
Gemstone 1.97– 1.91– 2.37– 2.73– 5.53–
Handbook 2.11 2.05 2.55 3.23 5.92

Article from the Rapaport Magazine - January 2007. To subscribe click here.

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