Boulder Opal, The Elusive And Hard-Won Treasure Of The Australian Outback
The vast arid expanse of the Australian Outback is a wilderness known for its stark beauty and ancient mysteries. This wild and unforgiving country is where determined miners tirelessly labour through tons of hard rock in search of one of the Outback’s most elusive and valuable hidden treasures- the boulder opal.
Considered to be one of the world’s most unique precious gemstones, the Australian boulder opal comes in a rainbow of scintillating colours like green and blue, but its red that is the most sought-after for its rarity and unsurpassed beauty. When any opal is viewed under moving light the colours within shift and change, making it no wonder that many cultures consider it to hold magical properties!
What is it that makes the boulder opal so special? Most boulder opals hail from Queensland and are mined in the regions around Yowah, Winton, and Quilpie. The boulder opal is usually discovered in the fractures and cracks in otherwise impermeable large blocks of ironstone and sandstone, hence its ‘boulder’ appellation.
While all types of opal typically display vibrant colours, the natural iron stone base of boulder opal provides it with extra stability and adds depth to the colour produced by its gems. The often odd and varied natural shapes that boulder opals occur in lend well to the fashioning of unique and unusual jewellery.
Queensland’s various opal-producing areas along with the stone layers in the formations can produce a variety of slightly different boulder opal types based upon the specific conditions under which it was formed. For example, the Yowah region often yields an opal that resembles a nut. A “Fairy” opal is produced when there is a light dusting of opal throughout the sandstone that is only visible when the stone is wet, creating the impression that “fairy dust” has been sprinkled upon it! This enchanting effect can be made permanent when the boulder opal is treated with a mixture of acid and sugar that has a darkening effect on the stone to provide contrast for the fairy dust colours to be highlighted against. This coating is then sealed with varnish to fix it into a durable gemstone perfect for the crafting of jewellery.
Boulder opals are found within crevices, cavities, and cracks in boulders that must be broken open. The gems are usually present in the clay layers above the ironstone. So, how is boulder opal formed? There are several theories, but no one is completely sure! One holds that over time the silica dissolved from sand trickled down into those cracks and crevices in ironstone boulders, then slowly precipitated out of solution. Another states that the presence of pyrite is required, and when it is dissolved by rainwater sulphuric acid is produced that dissolves the silica present in quartz. This silica reacts with feldspar to release more silica, then this colloidal silica fills up fractures in ironstone eventually solidifying into an opal. Yet another theory involves bacterial action, traces of which can be found in some opals. Every one of these theories involves the presence of groundwater, minerals, and sand.
Boulder opals are truly a thing of the stubbornly harsh Outback, a hard-earned treasure that can only be won by hard work!
