Modified: April 2011
How can two gemstones of apparently equal size vary in weight?
Diamonds, which are made of the single element carbon, all have the same specific gravity or density. However, unlike diamonds, colored stones are a mixture of multiple elements in varying combinations. Since all elements vary in atomic weight, so do the gemstones that contain them.
When dealing with diamonds, it is generally not necessary to mention millimeter sizes. A one-carat round brilliant diamond, for example, will always be cut within certain tolerances and, therefore, will generally be around 6 to 6.1 millimeters in diameter.
However, any other round gemstone weighing one carat may range from 5 to 8 millimeters, depending on the density or specific gravity of the stone. For this reason, when ordering castings for other colored gems, it is always important to designate the exact millimeter needed. A round one-carat cubic zirconia may only be 5.5 millimeters in diameter, making it somewhat undersized for a ring mounting that is designed for a round diamond weighing one carat. If you try to mount a one-carat fire opal in the same setting, it will probably be too large and, in most cases, cannot be made to fit.
So while many settings or semi-mounts may state that they fit a one-carat stone, they are referring to a diamond. All other stones need specific measurements for whatever shape they may take.
The second reason that colored gems need to be measured deals with the cut. Many gemstones are cut to maximize yield or weight from the rough, particularly expensive gems like ruby and emerald. When considering a casting for a colored stone, it is always a good idea to consider the depth of the stone as well as its width and length, or in the case of a round gem, its diameter. Some stones are also cut more deeply to intensify their color, such as tanzanite. When planning to set a gemstone, it is always advisable to bring the stone to your jeweler and ask for suggestions. A knowledgeable jeweler can help guide you through the selection process when ordering from a catalog or, if you prefer, help design a custom ring that incorporates your personal preferences.
Another point worth noting is that gemstones are small. While inches and feet do well for measurements around the house, most gems are measured to a tenth of a millimeter. This is very important when considering certain styles of settings that allow very little "wiggle" room for the stone. Although the English system of measurements feels more natural, with a little effort you can also master the metric system, which is the language of jewelers and gemologists.