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.
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.