A mineral that is found in many countries around the world, opal is more known as an Australian icon with 95% of the world’s supply found in our backyard. Opal mining happens all over Australia and has been mined since around 1889. In fact, Tullie Cornthwaite Wollaston first introduced the world to Australian opals when he took 60 pieces of opal from Australia to London, which began the great obsession with Australian opals.
There is so much to discuss on the geology, chemistry and history of this gem but the unique internal structure of opal, that is entirely responsible for its fabulous light show or lack thereof, is what I find to be the most interesting topic.
Opal is a hydrous, amorphous form of silica and is described as either precious opal or common opal (also called potch). Amorphous refers to a mineral without a clearly defined shape or form and so the material fills cavities as a liquid, solidifying in whatever negative space is available rather than crystallising in defined external forms such as prismatic quartz crystal. Common opal is abundant, translucent to opaque, single coloured material and is mostly used for backing of doublets and triplets unless it is characterized by interesting inclusions (dendrites). Precious opal has that deliciously intense ‘play of colour’, a kaleidoscopic display of light diffraction.
The formation of opal begins as silica particles within water-based solutions eventually merge together forming spheres as they naturally separate themselves from water. When the conditions are ideal these microscopic spheres, contained in silica-rich solutions from the each, merge and settle together in a void to form layer of silica spheres. An important factor for the ‘play-of-colour’ to exist is that the spheres must group to a size no smaller than 200 and no larger than 350 nanometres (a nanometre (nm) is one-billionth of a metre), which is exactly half the wavelength size of colour 400 – 700 nm. When the spheres’ sizes are the same and their stacking structure is orderly and symmetrical then best play-of-colour can be seen. As white light enters the opal it passes through the microscopic spheres and gaps splitting the light into flashes of vivid colour. Different sized spheres and therefore different sized gaps diffract different colours of the spectrum. Violet, blue and green are the more common colours and a red or a full spectral range being the most rare.
Precious opal can be white opal, black opal or boulder opal. Forming in a slightly different geological situation boulder opal is distinguished by a brown matrix of ironstone with delicate or striking ribbons of precious opal throughout. Discovered in Queensland in 1870 this state remains the only world location it can be found.
From my experiences over the year’s opal has been associated with souvenir purchases by tourists visiting our vast, dry land rather than something cherished by locals. Of course with everyone stone, there are lovers and collectors. However, opals are beginnings to gain more recognition on the international fashion scene with more of our Aussie starlets showcasing local designers and gemstones on the red carpet.
Bunny Bedi, owner and director at Made In Earth Creations, discusses his experiences with opals. “Opals prices generally reflect their colour and pattern rather than weight/size so they can be seemingly ‘all over the place’ and so as a local, or someone not in the industry, it can be difficult to know where to shop and what to look for with so many varying qualities, synthetics and imitations on the market.
“Recently MIE has been injected with some glorious opals and their response from our stockists and collectors has been nothing but positive. Set in simple, modern, sterling silver designs we’ve given these local stones a fresh look and it’s a competition amongst the fashion savvy and jewellery lovers to get the first pick.”
Opal will always resonate with us as an old-time classic Australian gemstone but with perceptions changing from un-cool to on-trend hopefully we can begin wearing our opals loud and proud.