Natural opals that exhibit a black body tone with abundant “play-of-color” are the most coveted.
Opal classification is uniquely complex. There are two broad classes of natural opal: precious and common. Three types of natural opal are: solid (homogeneous in composition), boulder (solid, but naturally attached to host rock) and matrix (diffused in host rock).
What determines market value is the base color (black, dark or light) and transparency (transparent, translucent or opaque). Opal can occur in every color of the rainbow, with black being the most prized. It’s ability to diffract light is distinctive, called “play-of-color”. Factors of this “play” include the body tone, the arrangement of color patterns (ideally large and tightly oriented) and the more vivid and wide-ranging the colors are, the better.
Like other gems, clarity and inclusions factor into market value. For clarity, this differs based on the kind of opal. For black opal, an opaque background exhibits the best play-of-color, whereas for crystal opal, transparency displays the best play-of-color. Opals with milky or cloudy appearance are less appealing to experts. Along the lines of inclusions, opals can have blemishes or contain fragments of its host rock, as seen in matrix opals. Often this affects the value, but it can also be something that is uniquely beautiful to the eye of the beholder.
While Australian opals are the most prized by collectors, opals can be found around the world. Below is a list of countries and brief descriptions of opals found in these locations.
AUSTRALIA: Australia produces the world’s largest supply of precious opals, including the most highly revered, rare black opal from Lightning Ridge.
ETHIOPIA: These hydrophane opals can be white, brown, yellow, orange or reddish in color with bright flashes of color. The best known are Welo opals from the Wollo Province. Always insist on disclosure, regarding if the opal is treated and untreated, as both exist in the market.
MEXICO: The most well regarded opal from Mexico is the fire opal. Their range is red, orange, yellow, or brown in body color and can be transparent to translucent. Also, they may or may not show play-of-color.
BRAZIL: These crystal opals vary from white, semi-transparent with unique pastel play-of-color to fully transparent body tone.
PERU: Known for their pink and blue common opal (versus precious), the stones are lovely even though they lack play-of-color.
OTHER COUNTRIES: Canada, Czech Republic, Guatemala, Hungary, Honduras, Indonesia, and the United States.
Opals can be treated to enhance their appearance and durability. Unlike other gemstones such as Sapphire, treating opals is not a commonly accepted practice. Natural Australian (apart from Andamooka Matrix), Mexican and Brazilian opals are very rarely treated. Ethiopian opals are the most treated from any location - and very unfortunately not always disclosed. All the more reason to buy from reputable sources and always ask if an opal is treated before purchase!
One treatment is a sugar treatment involving opals soaked in a sugar solution, followed by sulphuric acid. The process carbonizes the sugar, resulting in specks that blacken the body color and enhance the play-of-color.
Another treatment is smoking, which involves wrapping opals in paper, followed by heating and smoking them. Soot particles permeate the opal’s surface with the same result.
Other treatments involve plastic, resin, wax, colorless oil, and wax to minimize crazing.
When natural opal material is thin, it can be “assembled” to make it stronger. A doublet is a thin opal backed with glass, obsidian or other semi-precious material. A triplet is a thin opal sandwiched between domed colorless quartz and the same aforementioned backing.
Knowledge is power and any of the above treatment impacts market value.