Opal Cuts and Faults
In opals, there are 3 Cs that need to be payed attention to: Cut, Clarity, and Carat weight. Cut refers to the shape the stone has been cut into. Clarity refers to the severity of any inclusions found in the stone. Carat weight refers to the overal weight of the stone.
Generally, opals are cut into ovals, called cabochons, but they may also be cut into pears, rounds, marquise, freeform, sometimes, even faceted (see picture on left). Generally, cabochons cut opals are considered more valuable than the other cuts. When any stone is cut to a standard size, it is refered to as calibrated. When it is not, it's refered to as non-calibrated. The next factor that is taken into consideration is the dome of the opal. The degrees of the domes are high-dome, low-dome, and flat. When opals are cut, the edges should be rounded. Sharp edges make the stone likely to chip when it is set.
Other factors to consider are the smoothness and polish of the stones. A stone with scratches that can be seen only with the naked eye should be faulted. Generally, stones that are polished well, should have a high gloss, almost glassy look to them. If the surface has a wavy look on the surface, it should also be faulted.
Boulder opal cuts are judged a bit differently. To bring out the fire in boulder opals, it is generally necessary to cut an irregular surface onto the top of the stone. A boulder opal should only be faulted if the cut detracts from the overall attractiveness of the stone.
Cracks are aways faulted no matter where they appear on the stone. Like cracks in glass, they will grow over time, eventually causing the stone to outright break. To inspect for cracks, hold the stone to a light and look at the pattern. Any cracks that are in the stone will show up immediately. This is known as candling for cracks. This is important, as cracks in some opals will be internal, and thus won't show up in normal light.
Some opals will form tiny cracks on their surface that resemble the pattern of mud drying. This is called crazing. Unfortunately, there is no method of preventing opals from crazing. However, silica can be infused into the cracks of a crazed stone to make them nearly invisible. This treatment is only worth doing, however if the stone has a very high monitary value, or sentimental value. Crazing however, is nothing to really worry about, as only a very small percentage of stones ever craze.
J. Thomson Custom Jewelers uses only the highest quality opals, and all of our stones are guaranteed for one year against cracking or crazing. We are the only company in the industry to offer such a guarantee.
J. Thomson Custom Jewelers