You cannot weigh a gem in a setting and it is rarely worth removing. Instead, you can get a close approximation of the weight by the gem’s volume. In order to use these formulas, your measurements must be extremely accurate. Careful observation is also important. All these weights are adjusted by the stones proportions and it takes a trained eye to make those judgments.

There are two ways to approach this estimation. One can plug their measurement information into a program that calculates the weight for you, such as: DENDRTICS.COM:WeightEstimator,
or do it manually, as described below:

1. Measure the gem carefully in every direction. Record all your measurements and the stones shape, making special note of its proportions. Look up the specific gravity for the stone you are examining.
2. Apply the proper formula for the gem.

ROUND
Diameter X diameter X depth X SG X .0018

OVAL
Diameter X diameter X depth X SG X .0020
(Diameter = average of diagonal, length and width.)

SQUARE
Diameter X diameter X depth X SG X .0024
(Diameter = average of diagonal, length and width.)

SQUARE CUSHION
Diameter X diameter X depth X SG X .0018
(Diameter = average of diagonal, length and width.)

SQUARE step cut
Diameter X diameter X depth X SG X .0023
(Diameter = average of diagonal, length and width.)

SQUARE with cut corners
Diameter X diameter X depth X SG X .0024
(Diameter = average of diagonal, length and width.)

RECTANGLE
Length X width X depth X SG X .0026

RECTANGLE step cut
Length X width X depth X SG X .0025

RECTANGLE with cut corners
Length X width X depth X SG X .0026

RECTANGULAR CUSHION
Diameter X diameter X depth X SG X .0022
(Diameter = Average length and width)

PEAR
Length X width X depth X SG X .0018

HEART
Length X width X depth X SG X .0021

MARQUISE
Length X width X depth X SG X .0017

TRIANGLE
Length X width X depth X SG X .0018

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CABOCHONS

The actual range for flat-bottomed cabs is .0023 to .0029. Those with a bottom bulge range from .0024 to .0030. As a rule, use the following formulas:

High domed cabs
Length X width X depth X SG X .0026

Low domed cabs
Length X width X depth X SG X .0029

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The above formulas are for well-proportioned gems with a medium/thin girdle. Make the following adjustments as required.

GIRDLE
Use an average of the girdle thickness. Do not 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

On cut corner square and rectangle, wide corners can decrease weight as much as 5%.

Oval, pear, marquis, and heart, 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%.

The Curious Lore of Precious Stones:
by George Frederick Kunz

Gemstone Buying Guide: A Guide to Buying
by Renee Newman