Boulder opals also have a dark body tone, but they are not as valuable, while white opals have a light body tone and are generally the least valuable form of opal. There are a number of factors that alter the value of opals. As mentioned, black opal may have higher prices than light opal (especially with a black body tone like ink), although this is not to say that a fantastic light opal cannot be more expensive than a mediocre black opal. This is compared to the blackness scale shown below.
Stones that achieve values of N1 to N4 on this scale are considered black opals, which attracts the additional value associated with this class of opal. Visible inclusions may include; pot patches or lines, “straps”, “sand spots”, gypsum crystals and iron stone on the face of Boulder opal. The adjective solid has been removed from opal jargon for the simple reason that, from a scientific point of view, all varieties of opal are solid. Unlike its crystal family, quartz and agate, opal is not crystalline, which means that its composition is amorphous silica rather than crystal.
Here, the manager of Gem-A Instruments, Samatha Lloyd FGA EG, offers a quick but essential guide to the value factors of opal and what distinguishes a fantastic specimen from an average one. The range of transparency considered acceptable to define glass opal (transparent to semi-transparent) was taken directly from Robert Webster's discussion of transparency in his world-renowned textbook Gems. In addition, the polishing and smoothing of a cabochon allows the opals to better show their beautiful rainbow of colors. Natural light opal type 1 is the family of opal that shows a play of colors in or on a light body tone, when viewed face up, and can be designated N7, N8 or N9 on the body tone scale.
The Opal Association, as well as the entire industry, strongly supports the use of these standards to standardize nomenclature across the global opal industry. Most opals, especially black opals, tend to form as ovals, and because opal is cut “cabochon” (with a domed surface), these features have traditionally been preferred for aesthetic and jewelry calibration purposes. The best opals are black background color with a colorful, bright and evenly distributed play of color, free of any treatment and of Australian origin. If the body of the opal is transparent, also known as clear glass opal, you can see the spots of color below the surface.
Light opal is the most common variety found in most fields, but mainly in Coober Pedy and South Australia. The process Australian opal cutters use to value opal has been developed over many years using opal valuation software known as Smart Chart and uses principles that have been developed by industry representatives (Australian Gemmological Association GAA) and associations (The Opal Association) in recent decades. Opal type, body tone, brightness, pattern, color bar thickness, color play, and flaws play an important role in determining value.