Diamonds are cut to enhance the stone’s brilliance, dispersion or fire, and sparkle or scintillation. Other gemstones, for the most part, lack any real ability for ‘fire’, or the dispersion of light, and so are cut to focus on their brilliance and their sparkle. You should look at the shape for good symmetry and depth of the cut, evaluate for brilliance, identify any dark areas due to extinction, and look out for cavities in the gem body.
Shape or Outline: The gemstone you select should have a shape or outline that is well within the acceptable range. Symmetry of the shape is the most important factor. Remember that in order to retain maximum weight, a natural gemstone is almost never cut perfectly. In addition, a gemstone that is cut too shallow will create what is termed a ‘window’, through which you could literally read a word. It is not a desirable trait. Look for a well-balanced shape that is not cut too shallow.
Brilliance: As in diamonds, this is the most important factor in terms of cut quality, and it refers to the return of light, which shows up as bright areas present in a stone when viewed from top. A good quality stone will have its facets cut in order to maximize brilliance. The more sparkle it has, the better it is.
Extinction: Dark areas in a gemstone are known as extinctions. They appear as areas where no color is reflected back. A gemstone with a good cut will have a minimum area with extinction, or none at all. You should see as much color as possible.
Example of Extinction in the gemstone on the right
Another feature to watch out for is the presence of cavities. A cavity is a fissure or crack and is a common feature of a natural gemstone, especially in the pavilion, the area below the girdle. Cavities are something to avoid in any stone including diamonds, because their presence creates what is called an ‘optical void’. To solve this, cavities can be and are often filled like a tooth, or cleaned in order to reduce their appearance. While this might make them more visually appealing, once a gem has been tampered with in this way it is no longer considered ‘natural’, but is now considered ‘treated’, which affects its value. Also, if you purchase a stone that has had a cavity filled, be careful when cleaning it. Ultrasonic cleaning may dislodge the filling.
