Estimating Gem Weight

Compliments of
The International Gem Society

---

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.

When using this approach, careful measurements are a necessity. Careful observation is also important. All these weights are adjusted by the stones proportions and it takes a trained eye to make those judgments.

Step 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.

Step 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

---

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

---

Step 3

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%.