Calculate Gemstone Weight

It can be very difficult to calculate the exact weight of a gemstone when it is already in a setting, although this is something that many jewellers are frequently asked to do.

Antique pieces, family heirlooms as well as previously owned jewelry can all be challenging to value, but you can get a very accurate estimate based upon the gem’s volume.

Ideally, these calculations should be done by an expert as it requires a combination of both meticulous observations and precise measurements.

To calculate gemstone weight, all calculations must be adjusted according to the proportions of the stone.

To be able to evaluate these proportions takes years of training along with plenty of experience, with even some of the most accomplished jewellers often being unwilling to put their name 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 to do this as to the nearest millimetre is not accurate enough. Pay special attention to the proportions as well as the gem’s shape. You will then need to find the specific gravity (SG) of the stone.

#2. Use the correct formula for faceted gems

Naturally, every cut of gem has its own formula which needs to be used with even the slightest error meaning that your weight calculation will be inaccurate.

The calculations below are assuming 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 diameter of the stone 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 from between 0.0023 and 0.0029. If your gem has a “bottom bulge” the range is altered to between 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 make adjustments to 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%
  • 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%
  • An “obvious” bulge, add 9% to 12%
  • 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%.

How To Calculate Gemstone Weight 1