It can be difficult to calculate the exact weight of a gemstone when it is already in a setting, although this is something that many jewelers are frequently asked to do.
Antique pieces, family heirlooms, and previously owned jewelry can be challenging to value, but you can get an accurate estimate based on the gem’s volume.
Ideally, these calculations should be done by an expert as they require meticulous observations and precise measurements.
4 Ways To Calculate Gemstone Weight
To calculate gemstone weight, all calculations must be adjusted according to the stone’s proportions.
Evaluating these proportions takes years of training and plenty of experience, with even some of the most accomplished jewelers often being unwilling to put their names to their valuations.
The only alternative is to remove the stone from its setting. Although this may damage the jewelry, so is rarely worthwhile. So, how do you calculate gemstone weight?
#1. Measure the gemstone
Measure the gemstone in every direction. It is advisable to use an electronic device, as the nearest millimeter is not accurate enough. Pay special attention to the proportions as well as the gem’s shape. You will then need to find the stone’s specific gravity (SG).
#2. Use the correct formula for faceted gems
Naturally, every gem cut has its formula, which needs to be used with even the slightest error meaning that your weight calculation will be inaccurate.
The calculations below assume that the gems are correctly proportioned and have a thin to medium girdle. Should this not be the case, and your gem has imperfections or is abnormally sized, please see point four.
Unless you are dealing with round or rectangular cushions, you would need to calculate the stone’s diameter, which is done by averaging the length, width, and diagonal of the stone. All weights for gemstones are always in carats rather than grams or ounces.
Round stones
The calculation for round stones is diameter multiplied (x) by diameter multiplied (x) by depth (x) by SG multiplied (x) by 0.0018
Oval stones
Diameter x diameter x depth x SG x 0.0020
Square stones
Diameter x diameter x depth x SG x 0.0024
Square cushion stones
Diameter x diameter x depth x SG x 0.0018
Square step-cut stones
Diameter x diameter x depth x SG x 0.0023
Square stones with their corner cut
Diameter x diameter x depth x SG x 0.0024
Rectangle stones
Length x width x depth x SG x 0.0026
Rectangle step-cut stones
Length x width x depth x SG x 0.0025
Rectangle stones with their corner cut
Length x width x depth x SG x 0.0026
Rectangular cushion stones
Diameter x Diameter x depth x SG x 0.0022 (In this case, the diameter is calculated by taking an average of the length and width)
Pear-shaped stone
Length x width x depth x SG x 0.0018
Heart-shaped stone
Length x width x depth x SG x 0.0021
Marquise cut stone
Length x width x depth x SG x 0.0017
Triangle-shaped stone
Length x width x depth x SG x 0.0018
#3. Estimating the cabochon gem weight
For gems with a flat bottom, the cabochon (cabs) values range between 0.0023 and 0.0029. If your gem has a “bottom bulge,” the range is altered to 0.0024 and 0.0030. The formulas in these instances would be as follows:
High-domed cabs
Length x width x depth x SG x 0.0026
Low domed cabs
Length x width x depth x SG x 0.0029
#4. When adjustments are required for your gem weight
In many cases, you will be required to adjust your original calculations, especially when the stone isn’t well-proportioned. The necessary adjustments are:
Girdle
It is essential to get an accurate reading that you take an average thickness of the girdle. Never base your calculations on the bulge or point of the gemstone.
- On stones with a thin girdle, you should subtract 1% to 2%
- For stones with a slightly thicker girdle, you should add 1% to 2%
- For a thicker girdle, add 3% to 4%
- Very thick girdles, add 5% to 6%
- Excessively thick (seek confirmation), add 7% to 10%
Pavilion Bulge
- For a slight, add 3% to 5%
- For what would be regarded as a “noticeable” bulge, you should add 6% to 8%
- For an “obvious” bulge, add 9% to 12%
- For an “extreme” bulge, add 13% to 18%
- For stones that have a longer cutlet because of a steep pavilion, you can make adjustments of up to 5%.
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