It is difficult to evaluate opal because each stone is unique. In general, opals are rarer than diamonds because there are fewer high-quality opals available than diamonds. The good news is that you can find a gem-quality opal at a great price, especially when compared to a diamond. Some of the rarest opals in the world surpass even diamonds on a price-per-carat scale, such as The Eternal Flame, the rarest opal in the world.
Discovered in 1830 in the Ural Mountains of Russia, alexandrite has remarkable abilities to change color. Due to the small amounts of chromium in the crystal structure, this stone looks from emerald green to peacock blue in daylight, but from ruby red to purple under incandescent light. At that time, the red and green colors of Imperial Russia were all the rage. Therefore, it is not surprising that the Russian aristocracy coveted this stone.
This variety of chrysoberyl, named after Tsar Alexander, is still a rare stone. Although the discovery of alexandrite in Brazil and in some other places has expanded the availability of this jewel, it is still one of the rarest stones. Alexandrite, a modern June birthstone, is still popular and is often synthesized for use in jewelry. Taaffeite is one of the rarest precious stones, only about 50 are known to exist, which makes them about a million times rarer than diamonds.
Considering that the absolute value depends on the brightness, pattern and tone of the body; red and multicolored opals are rarer than green-orange, blue-green and blue opals in that order. To begin with, precious opal, potch and common opal are mentioned in the first section of the nomenclature. After much debate, the phrase body tone was added to the nomenclature to characterize the relative lightness or darkness of opal, regardless of its play of colors. Most stones look better in a particular orientation, some need to be tilted to appreciate them, while finer opals are not directional.
When Light passes through the surface of an opal, water breaks the Light into extraordinary wavelengths to produce the “fire that gives opal its price as a precious stone”. Consequently, any opal with a darker body tone than N4, regardless of the shade, can be correctly referred to as black opal. Despite any restrictions applied by this terminology, the term opal “jelly” will likely remain in colloquial use for many years. The Opal Association, as well as the entire industry, strongly supports the use of these standards to standardize nomenclature across the opal industry globally.
“In this context, the term “" crystal "” refers to the appearance of opal and not to its crystal structure.”. Today, the Australian Council of the Gemstone Industry and the Australian Gemological Association (GAA), the Australian Gem Industry Association (AGIA), the Lightning Ridge Miners Association (LRMA), the Australian Jewelers Association Ltd and the Opal Association have agreed, endorsed and published the following standards for the Opal appraisal process. This compares to the blackness scale shown below. Stones that reach values of N1 to N4 on this scale are considered to be black opals, which attracts the additional value associated with this class of opal.
For example, when buying a black opal, the nomenclatures establish confidence that the gemstone is actually “black” and not a minor stone such as white opal or crystal. When this happens, the best proof of transparency would be to “look through” the opal in transmitted light. Dropping jewelry with opals into an ultrasonic bath containing a caustic cleaning response is OK for diamonds, but can cause irreversible damage to opals. .