What is Opal?
In simple terms opal is a form of silica with traces of water trapped within the solid silica. While considered a gemstone opal is not considered a crystal due to its lack of a crystalline structure, making it a mineraloid. Solids lacking a crystalline structure are known as amorphous solids, which display most properties of a solid without having an organized atomic structure. Another well know amorphous solid is silica glass, the material used in drinking glasses and windowpanes.
While opal can consist of 1% to upwards of 20% water, stable opal consists of between 3% to 10% water. Australia's opal are believed to formed over 3-5 million years, with the majority of the opals found roughly 30 million years ago. The formation is believed to be caused by the Great Artesian Basin, the largest Artesian Basin in the world, being around 1,700,000 square kilometers, underlying 22% of the continent of Australia. The water slowly seeped into the ground, picking up silica from the sandstone. The silica was slowly redeposited in cracks deep underground. As the water dispersed it left only small traces of water within the silica, creating the opal we see today. This also lead to fossils becoming "opalized" where over millions of years opal replaces the fossilized object. Coober Pedy's "Shell Patch" is a great example of this phenomenon, where hundreds of pieces of opalized shells are found every day. Bones, teeth, shells and plant material have all been found opalized, including entire dinosaur skeletons.
The most well known types of precious opal are Australian opal, Mexican Opal, Ethiopian opal, and Brazilian Opal which each have their own unique properties.
Australian opal is a special type of opal known as sedimentary opal, which formed in the sandstone and mudstone during the Cretaceous age. While certain types of opal tend to craze as well as absorb water, such as most hydrophane Ethiopian opal, sedimentary opal is a much more stable form of opal, making it much more valuable. Opal that will not absorb water is also called non-hydrophane opal. Australia also produces 95% of the worlds precious opal and is the official national gemstone of the country.
Why Does Opal Display Color?
Opal is composed of hydrated silica (SiO2nH2O) which forms into colloidal silica spheres. To form precious opal these spheres must form in an orderly pattern as close together as possible, otherwise the light will not be refracted. Common opal, also known as "potch opal" lacks play of color for this reason, as it lacks the organized structure of precious opal. The size of the silica spheres also affect what wavelengths are diffracted, ranging between 150-440nm in size.
Types of precious Australian Opal
Precious Australian Opal is split up into different categories based on appearance; white opal, dark opal, black opal, boulder opal, and matrix opal.
Black opal is the rarest type of Australian opal. Due to its dark gray to pitch black bodytone, the flashes of color contrast more than on any other type of opal. Valued from $50 to over $15,000 a carat black opal is one of the most valuable gems in the world.
White Opal is the most well known of all types of Australian Opal. White opal is categorized by its light bodytone, appearing very light gray to a pure white. If you're looking for a top quality stone at a more affordable price, white opal is the way to go costing much less than black opal.
Dark opal is a rare form of Australian opal. Dark opal is categorized by its gray bodytone, directly between black and white opal. On the bodytone scale from N1-N9 (Black to white) dark opal is considered N5 and N6. Dark opal is more affordable than black opal while still being more expensive than white opal.
Crystal opal is one of the most brilliant form of opal. Classified by a translucent appearance, crystal opal isn't defined by the darkness of its bodytone. This transparency allows for a stunning 3D effect to take place, with patterns showing deeper into the stone than any other form of opal. While not as expensive as black opal many opal experts and collectors consider this to be their favorite type of opal due to this unique effect.
Boulder opal is a special type of opal from Queensland, Australia. This type of opal runs in thin veins through ironstone rocks. Due to this type of opal being found in such thin veins, it is left connected to the ironstone/host rock while being cut and polished. Boulder opal can have just as bright color as solid opal, while also costing significantly less.
Matrix Opal is similar to boulder opal in that is also connected to a host rock, but in a much more finer way. Matrix opal is essentially opal that is very finely distributed through the host, similar to that of cement, giving the stones a unique play of color. Matrix opal can also be treated with sugar mixtures to darken the base material, allowing the color to contrast and show up much more brightly.
Opal doublets and opal triplets are known as composite opals, which are composed of a layer of opal epoxied to another solid material to create one stone. Doublet opals are made with a thin layer of opal (usually crystal opal) with a layer of either Queensland ironstone, black opal potch (common opal with no color), or a synthetic black material such as a resin. Triplets consist of a very thin layer of opal epoxied between two layers; a dome made of clear quartz, and a black backing made from the same materials as doublets. The quartz dome creates a magnifying effect that gives the thin layer of precious opal a more 3D appearance, and protects it from cracking as well, due to it being such a thin slice of material.
Doublets and triplets are ways in which you can get an amazing bright colored black opal for a fraction of the price of solid black opals. Most commercial black opal jewelry that you can find on the market will consist of opal doublets and triplets, as high quality solid black opal is impossible to mass produce and would be much too expensive, with black opal costing up to $15,000 USD per carat.
Opal Doublets & Triplets
Australia's major opal occurrences (2016). Source: Geoscience Australia.
Australian Opal Evaluation
Australian opal evaluation is a complex process with many elements of the opal taken into consideration. The main attributes taken into account are bodytone, brightness (play of color), directionality, pattern, shape, polish, and faults (webbing, sand in face, inclusions, crazing, cracks, potch in face), while most gemstones use cut, clarity, color saturation, and tone. This wide range of attributes makes the process much more complex and requires years to fully understand and evaluate individual gems. This also explains the very wide price range of opals, ranging from $5.00 to $15,000 USD per carat.