Welcome to the International Opal Jewellery Awards Association - IOJDAA
Australia is the main producer of precious opal. Precious opal occurs in very few locations around the world because its formation would have required a series of geological, climatic and possibly biological phenomena to coincide.
This phenomena occurred in an area which today is the arid desert region that spans across Australia. Approximately 100 million years ago the same desert was an inland sea called the Eromanga Sea. This sea was teaming with marine life and laden with fine sand rich in silica. The receding of this sea marked the end of the Cretaceous period and formed the Great Artesian Basin. Deep weathering during the mid-Tertiary period approximately 30 million years ago caused changes to the sediments releasing large quantities of soluble silica. This silica filled cracks and cavities in the ground and when it eventually solidified it formed common opal (opal without colour). In rare circumstances precious opal was formed (opal with colour).
Soluble silica also filled fossil cavities. Some of the opalised fossils discovered have been of wood, coral, bones and other skeletal pieces. Opalised fossils with brilliant colour provide for valuable specimens and are considered to be amongst the most beautiful prehistoric fossils. Evolving over millions of years these fossils have become a very important part of Australia’s paleontological history and are sought after by collectors and museums around the world.
Types of Opal
The areas of opal production in Australia include Lightning Ridge, White Cliffs, Coober Pedy, Andamooka, Mintabie, Lambina, Opalton, Winton, Yowah and Quilpie.
The types of opal found in Australia are Light, Crystal, Dark, Boulder and Black. The most valued of all opal is Black Opal and is predominantly supplied by the opal fields of Lightning Ridge. Black Opal is precious opal which when viewed face up displays a play of colour on black potch (common opal with no colour). Precious Opal in Lightning Ridge is also naturally formed on potch that ranges from grey to black. Crystal opal produced in Lightning Ridge is translucent to transparent in appearance. The term crystal refers to the diaphaneity of the stone and not its structure. Precious crystal opal displays a play of colour within this translucent to transparent appearance.
Solid Natural opal is opal which has not been treated or added to in any way by mankind, other than by the process of cutting and polishing. Doublet opal on the other hand is a composite of two pieces. It consists of a slice of light opal glued to a base material. It is common practice for the base material to be natural common opal. Some examples of base material are black potch or boulder opal (opal formed naturally within its surrounding host rock). Triplet opal is a composite of three pieces comprising a thin slice of light opal glued to a base material and a protective clear dome glued to the top. It is common to use glass, quartz and plastic as the top layer.
Opals can be cut into different shapes. Conventional shapes include ovals, squares, rounds, rectangles, triangles and tear drops. Freeform on the other hand describes the shape of the stone which is any other shape than conventional. The Cabochon style of cutting as opposed to faceting refers to the dome shape of the stone. Undulating opal can be a conventional shape or freeform. Undulation gives reference to the surface of the piece. Carved opal has an undulated surface which is free in shape and may be figurative and representative to abstract. Undulating and carved opal helps conserve the weight of the opal and lends itself to unique and individualistic jewellery designs – IOJDAA’s charter.