What Does the L/W Ratio Mean in Gem Cutting?


Summary
L/W stands for “length-to-width.” Gem cutters use this mathematical ratio for determining the proper patterns for cutting rough. They also use it when creating new gem designs. The L/W ratio is one of the most important things gem cutters must understand when buying and cutting rough.
Reading time: 2 min 11 sec
oval-cut yellow sapphire - L/W ratio
This 0.90-ct oval-cut natural yellow sapphire has a L/W ratio of 1.413. © James Allen. Used with permission.

Measuring Rough Gems

First, measure the length, width, and depth of your rough, usually in millimeters. Of course, you don’t need the depth for the L/W ratio, but you’ll need it later in your design selection, so measure it now.

Next, take the length of your stone and divide it by its width. You’ll get a figure something like this:

10 mm/8 mm = 1.25

Thus, your L/W ratio for this stone is 1.25. It’s that simple.

Choosing a Gem Design Based on the L/W Ratio

Gem designs should include L/W ratios. For example, the L/W ratio for this Brilliant Marquise design is 1.401. If your rough has a ratio smaller than this, you can’t use it for this design. This design is too big for your stone. You’ll need to find a design with a smaller L/W ratio. You can always cut rough off and make a stone smaller, but you can’t add mass to a stone to fit a design.

Brilliant Marquise - L/W ratio
Brilliant Marquise by Jeff Graham.

Pick a design for your rough that has a L/W ratio equal to that of your rough. Or, if you don’t mind cutting off some weight from your gem, pick a design with a ratio slightly smaller than that of your stone.

Use the following chart to help you choose a design for your rough. You’ll need to round down to the nearest ratio that fits your stone.

Standard L/W Ratios

Ovals / Rectangles Pears Marquise
3 x 5 = 1.67 3 x 5 = 1.67 2 x 4 = 2.0
4 x 6 = 1.50 4 x 6 = 1.50 2.5 x 5 = 2.0
5 x 7 = 1.40 5 x 7 = 1.40 3 x 5 = 1.67
6 x 8 = 1.33 5 x 8 = 1.60 3 x 6 = 2.0
7 x 9 = 1.29 6 x 9 = 1.50 4 x 8 = 2.0
8 x 10 = 1.25 7 x 10 = 1.43 5 x 10 = 2.0
10 x 14 = 1.40 6 x 12 = 2.0
12 x 16 = 1.33 7 x 14 = 2.0
13 x 18 = 1.38

Keep these equivalencies in mind:

  • Ratio = length divided by width
  • Length = ratio times width
  • Width = length divided by ratio

Non-Standard L/W Ratios

You can cut a non-standard L/W ratio if you have a design that calls for it. You can also create a design that uses one. However, most standard jewelry settings are calculated for standard gem sizes, so they won’t work with your design. You’ll have to create a custom setting for it.

About the author
Jeff R. Graham
The late Jeff Graham was a prolific faceter, creator of many original faceting designs, and the author of several highly-regarded instructional faceting books such as Gram Faceting Designs.
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