Australian Boulder Opal Direct From The Cutter
Queensland Boulder Opal.
Queensland boulder opal is a solid opal consisting of a vein of coloured opal with a natural ironstone host rock.
The Queensland opal fields extend across a wide area through central Queensland. The opal from each specific locality has its own unique character ranging from Yowah nuts with stunning matrix patterns to great boulders with veins of gem quality opal from Quilpie to thin veins of striking colour in pancake boulders near Winton.
Boulder opal is found in a layer of soft sandstone, sedimentary layers that were once the seabed of the ancient inland sea called Eromanga. Within this layer, pockets of sandstone have formed with a higher concentration of iron to create the harder and darker boulders. At some stage during their formation, these boulders have internally cracked and crazed to allow a silica compound to fill these cracks and then harden, forming the veins of opal that we call Queensland boulder opal. The opal vein is cut to expose the colour and retain the ironstone backing.
Black opal is found in Lightning Ridge in New South Wales. Black opal is a solid opal consisting of a coloured band of opal with a natural backing of a band of black or grey opal. This dark backing layer can vary from light grey to black, the darkest backing colour resulting in the most intense play of colours in the opal.
White Opal is mined in South Australia in Coober Pedy. This opal generally has a milky white appearance, without the same intensity in the play of colour that a black or boulder opal can display. White opals are commonly opalised shells from the ancient inland sea bed.
White Cliffs is a town in New South Wales where a limited quantity of white opal has been mined. Andamooka is a South Australian town where a matrix style of opal is mined with the opal occurring in a conglomerate form with the host rock.
The microscopic structure of the opal consists of tiny spheres of silica, held in place by a layer of hydrated silica. The size of the spheres determines the wavelength of light that is seen in the opal and therefore the colour. Gem quality opal with intense colour is the result of uniformly aligned silica spheres in a highly organised structure, whereas randomly arranged spheres results in dull coloured opal with little value, commonly referred to as "potch". There are vast quantities of opal in the Australian outback, but only a very small portion of that opal has colour that would be considered of any commercial value, with gem quality pieces being very rare indeed.
About opal cutting. Cutting a gem quality opal begins at the mine site, where the boulders are pulled from the layer of soft sandstone. Boulders can range in size from the size of a tennis ball, up to 2 metres in length and weighing several hundred kilograms. The boulder is split open to see if there are any potential veins of colour inside. From there, the boulder is cut using a brick saw into smaller blocks that can be further sliced using a small diamond saw. This stage of the process is exciting, as the opal cutter determines the best way to cut the boulder to reveal the three dimensional puzzle of opal concealed within. The veins of colour can be several millimetres thick or as thin as a hairline, with the challenge being to expose one surface of the opal vein to show the face of colour. Fine diamond grinding wheels and sandpaper are used to carefully remove the sandstone from one side of the opal vein, leaving a layer of opal with its natural ironstone boulder backing, ready to be polished and made into jewellery.