Opal is the national gemstone of Australia, where the vast majority of the precious variety of this stone is mined. It is known for the striking display of shifting rainbow colors that the finest specimens exhibit, called play-of-color. No two opals are alike, from their base color to the hues found in their play-of-color, which means that determining the value of an individual stone is less straightforward than for other gemstones.
Take diamonds, for example. To determine the value of a diamond, you take into account the four Cs—color, clarity, cut, and carat weight. Evaluating an individual stone on these primary factors allows you to easily compare it to other diamonds. Opals, on the other hand, should be evaluated not only on the size of the stone and how skillfully it is cut, but on the base color, the stone’s clarity, and the colors and pattern of the play-of-color. Each of these factors will influence the value of the stone.
In general, opals tend to be cut in cabochons, which best display the shifting colors for which it is known. (The exception to this rule is fire opals, which may also be found in faceted cuts.) A jeweler may cut an opal into a free-form shape to best display an unusually striking play-of-color, but in general, symmetrical cabochons that are neither too thin nor too thick are preferred, and will be valued more highly.
The base color of an opal can range from white to black, with a white or light background being most common. Darker stones are preferred, as they tend to show a more vivid and thus more desirable play-of-color. Black opals, which have a jet-black base color, are the most valuable, and can go for thousands of dollars per carat, depending on the other factors affecting a stone’s overall appearance.
Opals can be found with a clarity from transparent to opaque, with greater transparency increasing the value of the stone. Clear opals of any base color are called crystal opals, and are coveted because their transparency makes the play-of-color throughout the stone more visible. Inclusions are evaluated solely on how they affect the overall appearance of the stone.
Many factors are considered in evaluating the play-of-color itself. First, the play-of-color should ideally be centered on the stone and not include any dead spots (i.e., places in the stone that exhibit no play-of-color). It should be bright and visible under any lighting conditions to command the best prices. How many and which colors are included in the play-of-color matter—colors on the warm end of the spectrum are rare, and a more varied display-of-color is better than a limited one. Finally, larger or rare patterns of play-of-color are more valuable than small patterns that may show only pinpricks of color across a stone.
Ultimately, the value of an individual opal is somewhat subjective—the beauty of each opal is unique to that particular stone, because of the geologic processes that form it. To find precious opals at below-market prices, visit the online auction at Auction King. Our live auctioneer can answer any questions you have about individual pieces via our chat module, so you can feel confident you know the characteristics of the necklace or ring you choose before you buy. Sign up for a free account to get started.