Opal Explained
Introduction to Opals
Incredibly rare, beautiful and captivating Opals are the #1 most valuable commercially available gemstone on earth, Australia’s National Gemstone and the NSW state emblem. The name “Opal” is derived from the Sanskrit Upala “Precious Stone” and the Latin Opalus: “to see a colour change”.
Opal has been revered throughout the ages and across the continents with Opal deposits in the Middle East. In the first century, it is believed that Hungarian Opals (from Cervenica or Cenowitz) were cut in Idar-Oberstein in Germany and traded back through the silk road (so that the Romans would not invade Hungary and plunder their wealth). Opals have also found their way (through mining or trade) to Mexico, Honduras and Aztec Peru.
Set with translucent White Opals, the “Crown of Charlemagne” once adorned the head of the Holy Roman Emperor in the 10th Century. Albert Magnus described the incredible “Orphanus” Opal centre stone: “None like it has ever been seen... Its hue is as though pure white snow flashed and sparkled with the colour of bright ruddy wine...it is a translucent stone, and there is a tradition that it shone at night-time”.
Holding a rainbow
Australian Opal behaves similarly to a rainbow, except that the water is still rather than flowing. Every other gemstone relies on faceting or chemicals to produce rainbow colours. But Australian Opal forms when water (H20) bonds with Silica (SiO2N2) to create a new unique, incredible gemstone we know and love as Opal (SiO2N2H2O). Literally, Billions of molecules have to form in perfect alignment in order for the spectacular rainbows to form. If the molecules are scattered, then there is no colour (we call this Opal-without-colour "potch").
When you hold an Opal, you are essentially holding a fossilised rainbow! Watch
Opal is created by silica-containing fluids and may be discovered inside any type of rock. At low temperatures, silica gel precipitates to form layers or nodules of Opal in fissures, veins, and cavities in volcanic and sedimentary rocks all over the world. Opal is an amorphous silica (SiO2.nH2O) that is chemically identical to quartz (Si02). But quartz contains 3-21% water within its mineral structure, while pals of gemstone quality usually contain 6% to 10% water.
Precious Opal is distinguished by its characteristic "play of colours", the way colours shift inside a certain stone as it is rotated and tilted. Common Opal, sometimes known as "potch," is normally colourless or white, but fine-grained impurities can turn it grey, brown, red or yellow. Potch accounts for 80-90% of all Opal and has little economic value.
Diffraction is responsible for the colour play found in Opals. Water percolates through the soil material under the right conditions. Silicates found in the soil dissolve in this water, resulting in a silicate-rich solution. When water enters a cavity, it deposits silicates as microscopic spheres. Precipitated silica spheres layers form a jelly-like water mass, which can occasionally produce a diffraction prism when the spheres are uniform in size and order. The diffraction grating design produces a rainbow of glittering light from within the stone.
The colour play of opal is completely owing to the regularity of small spheres, each about a tenth of a micron in diameter. Common Opal is generated when the spheres are randomly shaped and arranged. They will diffract light if they are consistent in size and shape, and the colour play will be seen. The colours produced by the Opal's interior structure of regularly packed spheres vary depending on the size of the spheres and the spaces between them. When you move the stone, light strikes the spheres from different angles, causing the colour to change.
Depending on the opal’s sphere size has a bearing on the colour produced. Blues and violets from one end of the spectrum are enhanced by smaller spheres (less than roughly 150 nm). Oranges and reds are produced by bigger spheres (no greater than 350 nm). Because these spheres are so tiny, the size difference equates to nearly 3 million "bigger" spheres and 6.5 million "smaller" spheres lined up within the area of a millimetre. The colour will be more vivid, dazzling, and defined if the spheres are of consistent size.
Fossilised Dinosaurs
Australia is the only part of the world where Opalised animal and plant fossils have been found.
The Opal fields of Australia were originally an inland sea. Sea animals were stranded and marooned as the centuries passed and the oceans receded, and Opalised. The land eventually dried out entirely and is now dry desert land. Groundwater containing silica solution evaporated over time (with some artesian springs still active deep "underground"). Opal is generated when water runs down the soil and picks up silica from sandstone. This silica-rich fluid is subsequently moved into natural faults or disintegrating fossils-caused fissures and voids. As the water evaporates, a silica deposit forms. The deposit ultimately hardens to produce ordinary Opal, and in exceptional cases, it creates precious Opal.
At Lightning Ridge in NSW, small Opalised dinosaurs and primitive early mammalian remains, together with shallow marine shellfish and crustaceans have been found. Probably the most famous Opalised fossil is Eric the Pliosaur (Cretaceous age marine vertebrate) which was found at Coober Pedy and now forms part of the Australian Museum collection.
Spectacular mineral replacement with Opal can also occur. The so called 'Opal pineapples' found at White Cliffs, NSW are Opal pseudomorphs of the mineral glauberite are examples of this. Queensland Opal deposits occur in non-marine sedimentary rocks and Opalised animal remains are very rare, however fossil wood fragments showing annual growth rings and cellular wood textures occur and sometimes these are beautifully replaced with precious Opal.
Gem black opal is 5000x rarer than diamond
Opal is a very valuable gemstone. Black Opal is potentially 130 times rarer than diamond! Only one million carats (or 200kg) of opal are mined for every 130 million carats of diamond produced. And the cutting loss factor from this rough might be as high as 95%. (In black Opal).
"Tanzanite is 1000 times rarer than diamond," you may have heard because there are 100 miles of tanzanite mines throughout the world, and when compared to the 100,000 miles of diamond mines across the world, "tanzanite is 1000 times rarer than diamond." Because there are only 20 square miles of black Opal mining, black Opal is 5000 times rarer than diamond.