How to Estimate Gem Weight in a Jewelry Setting


gem weight - brooch
“Cut Gem s vi,” sapphire and diamond brooch, by gini. Licensed under CC By-SA 2.0.

Although you can’t weigh a gemstone in a jewelry setting, you can get a close approximation of the gem weight by determining its volume.

When using this approach, you must take careful measurements and observations. The weight you’ll calculate must be adjusted by the stone’s proportions. Making those judgements takes a trained eye. The alternative, removing and resetting the stone, is rarely worthwhile.

Step 1: Measure Your Gem

  • Measure the gem carefully in every direction.
  • Record all your measurements as well as the stone’s shape. Take special note of its proportions.
  • Finally, look up the specific gravity (SG) for the stone you’re examining in our gem listings.

Step 2: Formulas for Estimating Faceted Gem Weight

Apply the proper formula for the gem based on its cut. The following formulas assume you’re dealing with well-proportioned gems with a medium/thin girdle. If that’s not the case, you’ll have to adjust your results. See Step 4 below.

For all cuts except rounds and rectangular cushions, calculate the diameter as the average of the diagonal, length, and width.

Weight estimates are in carats.

Round

Diameter X diameter X depth X SG X 0.0018

Oval

Diameter X diameter X depth X SG X 0.0020

Square

Diameter X diameter X depth X SG X 0.0024

Square Cushion

Diameter X diameter X depth X SG X 0.0018

Square Step Cut

Diameter X diameter X depth X SG X 0.0023

Square with Cut Corners

Diameter X diameter X depth X SG X 0.0024

Rectangle

Length X width X depth X SG X 0.0026

Rectangle Step Cut

Length X width X depth X SG X 0.0025

Rectangle with Cut Corners

Length X width X depth X SG X 0.0026

Rectangular Cushion

Diameter X diameter X depth X SG X 0.0022. (NOTE: diameter = average of length and width).

Pear

Length X width X depth X SG X 0.0018

Heart

Length X width X depth X SG X 0.0021

Marquis

Length X width X depth X SG X 0.0017

Triangle

Length X width X depth X SG X 0.0018

Step 3: Formulas For Estimating Cabochon Gem Weight

Flat-bottomed cabs actually range from 0.0023 to 0.0029. Those with a bottom bulge range from 0.0024 to 0.0030. As a rule, use the following formulas.

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

Step 4: Adjusting Your Gem Weight Estimate

If the stone you’re examining isn’t well proportioned or has a very thin or thick girdle, adjust your gem weight estimate as follows.

Girdle

Use an average of the girdle thickness. Don’t base your reading on a bulge at the top of a heart or the point of a gem.

  • Thin girdle, subtract 1% to 2%.
  • Slightly thick, add 1% to 2%.
  • Thick, add 3% to 4%.
  • Very thick, add 5% to 6%.
  • Extra thick, add 7% to 10%.

Pavilion Bulge

  • Slight, add 3% to 5%.
  • Noticeable, add 6% to 8%.
  • Obvious, add 9% to 12%.
  • Extreme, add 13% to 18%.

A long culet due to steep pavilion angles can add up to 5%.

Shape Outline

  • Wide corners on cut corner squares and rectangles can decrease weight as much as 5%.
  • For oval, pear, marquis, and heart cuts, wide wings or high shoulders can add up to 10%. Occasionally, straight shoulders will require a deduction of 1% to 5%.
  • Marquis and sometime pears will have a very short keel or none at all. This will reduce the weight by 1% to 3%.
  • Triangles with straight sides will require a reduction of up to 10%.
gem weight - ring stone
“Sunflower Fire Agate Ring” by Jessa and Mark Anderson. Licensed under CC By 2.0.