Opals are basically a combination of silica and water. More correctly, opals are a silica gel, with varying percentages of water and this percentage is normally around 2-6% but could be considerably more. Due to this factor, opals may easily become brittle and if stored too dry or exposed to heat over a longer period of time, opals will show fissures and the play of color will become paler if they are stored too dry or exposed to heat over a long period of time. Interestingly this means that opal jewelry should be worn as often as possible as the gemstone will receive the needed humidity from the air and from the skin of its wearer.
Opals are not very hard: they only achieve 5.5 to 6 on the Mohs’ scale. Therefore they appreciate a protective setting. In earlier days opal’s sensitive surface was often oiled, but today also sealing them with colorless artificial resin has become quite popular.
Almost ninety-five per cent of all fine opals come from the dry and remote outback deserts of Australia. The remaining five per cent are mined in Mexico, and in Brazil’s north, also in the US states of Idaho and Nevada, but recently the stones have also been found in Ethiopia and in the West African country of Mali.
There are many legends surrounding this colorful gemstone and one takes us back to the ancient dream time of the Australian aborigines. In their legends, it is reported the creator came down to Earth on a rainbow, in order to bring the message of peace to all the humans. And at the very spot, where his foot touched the ground, the stones became alive and started sparkling with all the colors of the rainbow - that was the birth of the opal.
Opal’s color play emanates a very special attraction and fascination. This play of color is called “harlequin”, “church windows”, “needle fire” “flame opal” or “lightning and peacock opal”. It was only in the 1960s that a team of Australian scientists discovered that small spheres from silica gel caused interference and refraction manifestations, which are responsible for the fantastic play of colors. The spheres, which are arranged in more or less compact hexagonal or cubic structures, succeed in dissecting the light on its passage through the gemstone and turning it into all the colors of the rainbow, always new and always different.
There are many different kinds of opal - dark or black opal white or light, milk or crystal, boulder opal, opal matrix, Yowah Nuts from Queensland (also known as picture stones), and Mexican and fire opal. All the opals, with the exception of fire opal, show the unique play of colors. If an opal does not have the play of color, they are known as common opal.
In 1849 the first opal blocks were accidentally found on an Australian cattle station called Tarravilla and the first opal prospectors started in 1890 at White Cliff. The other legendary sites of the Australian opal fields are Lightning Ridge, Andamooka or Coober Peddy. The most famous one is probably Lightning Ridge, where the famed black opal is found. Andamooka, where crystal opal and light opal are mined, is where the Andamooka Desert Flame opal was found, currently the largest opal with a weight of 6,843 kilograms. Coober Peddy is a word from Aborigine language meaning “white man in a hole“ which clearly describes how opal was mined. Many opal prospectors made their home in holes or caves in the ground to protect themselves from the burning heat of daytime and from the cold night winds. They worked with picks and shovels and buckets of soil, hopefully containing opal, were pulled up from depths of 15 to 120 feet as this is where the opal was located.
Nowadays there are some technical means available, such as trucks or conveyor belts but the thought of finding that special opal still lures people to the mines.
The stones are cut and polished to round or oval cabochons, or any other softly domed shape. Only the best qualities of fire opal are suited to faceting. Before working on the rough shape, the opal cutter will remove any impurities using a diamond cutting wheel, will fine cut, finish with sandpaper and then the final polish with a wet leather wheel.
The most important criterion for determining the price of an Opal, however, is the play of color, the colors as such and their pattern. If the color red appears when looking through the stone, all the other colors will appear also. For evaluating Opals the thickness of the Opal layer is considered, the beauty of the patterning, the cut, weight and finish. Finally the total impression will be decisive, and of course offer and demand will determine ho much you will have to pay for “your” Opal. If you are interested in a really valuable specimen, get an Opal expert to advise you, because it takes a real expert to know about the many criteria which determine the price.
(Photographs and text courtesy of the International Gemstone Association and Wikipedia).