Once categorized, opals are sold in price by carat or weight. Because there are so many fields of opal in Australia, there is really no single form of opal. When considering colors, a layer of secondary and more complex value emerges. The “dominant color” of an opal can affect its value, with red having the highest cost, followed by orange, yellow, green, blue, indigo and violet.
However, do not forget, a bright blue-green stone will be more valuable than a dull red one. 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. The thickness of the color bar in opal is relative to the overall size and shape of the individual stone. Hyalite and precious opal that are completely transparent, colorless and rich in fire are found in San Luis Potosí.
Opal is worth a lot because of its unique phenomenon of play of colors, which cannot be observed in any other gemstone. A triplet adds a transparent quartz top and is a good ring stone, because hard quartz prevents softer opal from being scratched. Solid opal that is translucent to opaque when viewed from above and that has a play of colors against a dark gray background that is classified as semi-black on the Lightning Ridge Miners Association tone scale. It is caused by the diffraction of white light through the internal structure of ordered silica spheres and produces the brilliant spectrum of colors by which opal is known.
If an N9 opal (which would be a white opal) has a magnificent color and brightness, it will exceed 10 times the value of N1. The term crystal opal refers to the “diaphanity” (transparency) of an opal, not its crystal structure, and is defined as any type of opal that is translucent or transparent. Some opal patterns are common and don't add much value, while rare patterns send it through the roof. Hydrophane opals absorb water and chemicals, which can have a negative effect on their appearance. Australian vernacular names for opal and phrases that have been part of the Australian landscape for hundreds of years have contributed to the mystery and mythology of the daily language used in opal mining fields.
The color segment pattern, which forms the play of colors of a precious opal, is unique to each individual opal. Any indication of the “origin” of an opal (by use of geographical location), is not used unless qualified as an indication of the type of locality as recommended by the International Confederation of Jewelry, Silverware, Diamonds, Pearls and Stones (CIBJO) e. Gem cutters can take opal pieces too thin to use as solid gemstones and mix them into doublets and triplets.