The distribution and intensity of these colorful flashes is what determines their cost.
Opal stoneis mainly found in Australia, where mines supply almost every opal in the world. This is another factor that makes the gemstone more expensive compared to the others. Not all opals are valued at the same price.
Australian opals are incredibly expensive, especially compared to opals from other regions of the world. The reason is that Australian opals have a superior quality that is difficult to get in other regions of the world. Are there opals available in other areas of the world? You can bet. While Australia gets 90% of the world's supply of opals, you'll also find them available in Ethiopia, Mexico, Brazil, Indonesia, and even the US.
In the US, American opal is a rare gemstone because its formation requires a very specific environment.
Opalis so valuable for its unique phenomenon of play of colors, that it cannot be observed in any other gemstone. Opal is very expensive because each stone has its own unique pattern. There are no two similar opals.
Black opal is one of the most charming stones in the world, sought after for its seemingly infinite display of colors. And there's a whole epic story of why there should be opal there, and you go and dig there, and it's not. Therefore, the most valuable black opal of all is a very bright red stone on a very black base that simply shines. Red opal (fire) is generally more valuable than a mainly green opal which, in turn, is more valuable than a stone that shows only blue color.
Compared to ordinary opal, which is usually single color, black opal exhibits many different colors in contrast to a dark body tone. The distinctiveness and color shown by these segments determine the quality of an opal's pattern. Valuable opal is the one with a black background color; the play of colors contains shades that span the entire spectrum; the sparkles are large enough and are evenly distributed throughout the gemstone; no inclusions or fractures. That exclusivity and unique colors of black opal make it one of the most expensive gemstones in the world.
The type of opal, the tone of the body, the brightness, the pattern, the thickness of the color bar, the play of colors and the faults play an important role in determining the value. All of the above factors are taken into account when valuing opal, however, there is no substitute for experience. While Australia gets 90% of the world's supply of opals, you'll also find them available in Ethiopia, Mexico, Brazil, Indonesia, and even the United States. Let's say that the larger opal is less iridescent and looks much more hazy than the small opal on the right hand.
Australian opals are found in some of the most dangerous and isolated parts of the continent, making the hike to extract them tricky. Although this opal is smaller, it reflects light and has a better play of colors than the larger opal.