Why Australian Opal is Special?

The opal is the National Gemstone of Australia. Australia currently produces approximately 95 per cent of the world's precious opal and probably has almost all of the world’s opal reserves. The only other significant producers are Mexico and Brazil although the deposits in Slovakia and the Czech Republic once provided the bulk of the world’s production for over 2,000 years. Other countries where opal has been recorded include Guatemala, Honduras, Nicaragua, the western USA and Canada, Indonesia, Turkey and Ethiopia.

Opal forms in cavities within rocks. If a cavity has formed because a bone, shell or pinecone was buried in the sand or clay that later became the rock, and conditions are right for opal formation, then the opal forms a fossil replica of the original object that was buried.

Intense weathering resulted in extensive silicification at relatively shallow levels within the Tertiary regolith. However, despite a billion dollar industry and a well-constrained geological history of the basin, the formation of sedimentary opal and its uniqueness to the Australian continent are still very poorly understood.

The sedimentary opal deposits of central Australia occur along generally flat-lying horizontal layers within 30 meters of the earth’s surface. They are a product of a unique set of geological events which occurred over a 100 million year period. 

A unique combination of geological events appear to have taken place in central Australia over the last 100 million years to form precious opal. These are:

1. Deposition of volcanic-derived organic-rich sediments over a 30 million year period during the Cretaceous.
2. Weathering under warm wet acidic conditions from the Late Cretaceous to the mid-
   Eocene which released silica and iron.
3. Remobilization of the silica under warm arid alkaline conditions during a period of
   tectonic instability during the Late Oligocene-Early Miocene.
4. Preservation of the opal forming in the weathering profile through lowering of the water table.

Deposition of precious opal occurs mostly by replacement of layer-silicate clays, gypsum, calcite, goethite, fossils and organic material as well as by infilling of voids particularly in ironstone.

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