The South Australian Museum said the stone, valued at more than AUD1.0 million (USD730,000), would go on public display for the first time in September to mark the centenary of opal mining in the country.
"It's of unequalled quality, it's a fully crystal opal," museum director Brian Oldman told AFP.
"It's almost as if there's a fire in there; you see all different colours. As the light changes, the opal itself changes. It's quite an amazing trick of nature."
Some 90 per cent of the world's opals come from South Australia, once covered by an inland sea which over millions of years provided an ideal environment for the formation of the stone.
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"I think this exhibition will have the finest collection of precious opals that we believe have been brought to one place in the world," Oldman added.
Opals were first discovered at Coober Pedy -- widely-known as the opal capital of the world -- in 1914 by a boy named Willie Hutchison who was on a gold mining expedition with his father.