How is Opal formed?

The uniqueness of the opal extends further than just the physical stone that miners toil for and magnificent stones you see on display in opal shops and the stunning pieces created by specialist jewellery artisans. The creation of the opal is so unique that requires a special series of specific geological, climatic and biological elements to coincide to enable the formation of this elusive gem, and as such, it is only found in a few places around the world. Australia is fortunate to be one of those places and our central desert regions account for around 90% of the international opal supply.

These phenomena that contribute to the formation of opal extends back to the Cretaceous period (145-65 million years ago) and during that period the interior of Australia was made up of an inland sea and towards the end of this era, the water recede, refilled, and receded several times and continued to do so for may aeons, resulting in deposits of fine marine sands rich in silica being deposited on the ever-changing shoreline. About 30 million years ago, extensive weathering acting upon the stratified sediment and released large quantities of soluble silica.

Voids and cracks in the ground enabled the silica-rich solution to flow down and these are where the opal forms. Additionally, as organic matter of plants and animals was laid down into the sediment during the Cretaceous period and that in turn decomposed, the solution also filled these spaces to form the much prized fossilised opal. This process is very slow and is estimated by experts to take as long as 5 million years to form a 1cm thick opal vein.

The resultant gemstone is a non-crystalline silica, similar to quartz, but is not a mineral. Its internal structure enables unique diffraction of light to produce white, grey, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black.

Due to the unique nature of its formation, there are no steadfast rules of reasons how and where it is formed and as such, Opal mining is not an exact science. Many people, especially miners, equate opal mining as playing the lottery, but with a lot of digging.