Have you ever wondered why two gemstones of apparently equal size vary in weight?
Diamonds, which are all made of the single element carbon, have the same specific gravity or density. However, unlike diamonds, colored stones are a mixture of elements in varying combinations. Since all elements vary in atomic weight, so do gemstones.
Generally, when dealing with diamonds, it is not necessary to mention millimeter sizes. A one carat round brilliant, for example, will always be cut within certain tolerances and will generally be around 6-6.1 millimeters in diameter. However, a round gemstone weighing one carat may range from 5 to 8 millimeters, depending on the density or specific gravity of the stone.
When ordering castings for colored gems, it is always important to designate the exact millimeter needed. A round one carat CZ may only be 5.5 millimeters in diameter and will be somewhat undersized for a ring that is designed for a round diamond weighing one carat. If you try to mount a one carat fire opal in the same casting, it will probably be too large and in most cases cannot be made to fit.
So while many castings or semi-mounts may state that they fit a one carat stone, they are referring to a diamond, the “king of gems.” 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 or 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, the diameter.
Some stones are also cut more deeply to intensify the color a very common feature, for example, of tanzanite. When dealing with 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 catalogue or, if you prefer, help design a ring that incorporates your personal preferences.
Another point worth noting is that gemstones are small. While inches and feet do well 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 all jewelers and gemologists.