Written by By Georgia McCafferty, CNN

The world's finest uncut opal, a 998-gram gem called the Fire of Australia , has gone on permanent public display for the first time since its discovery more than 60 years ago.

Valued at more than $675,000, the stone is the largest known high-grade opal in the world, according to the gem's new owners, the South Australian Museum in Adelaide, Australia.

"The Fire of Australia is around the size of a softball...and shows all the colors of the spectrum," museum director Brian Oldman told CNN, emphasizing the extreme rarity of the stone.

"The opal is unique because of its high quality and large size. It throws off a lot of red, which is one of the indications of a highly valuable piece of opal."

Desert find

The gem was first discovered in 1946 by miner Walter Bartram at the Eight Mile opal field in Coober Pedy -- a small desert town in South Australia famous for its opals.

(South Australia, which encompasses a vast arid area in the south and middle of Australia, produces more than 90% of the world's precious opal, according to Oldman.)

Oldman said it would have been part of a larger seam of opal that ran underground, and would have been extracted in pieces.

Recognizing the uniqueness of the piece, the Bartram family, who own an opal mining and distribution business, held onto the piece.

"The Bartrams polished two sides of the Fire of Australia, as they recognized that it was a significant discovery and wanted to show the quality of the opal," Oldman said.

"Usually a large piece of opal doesn't get polished at all. Instead, it gets cut up for jewelry and then it gets polished."

The stone remained in the family for more than 60 years, spending most of that time in a safety deposit box. But after loaning it out to the South Australian Museum for an exhibition, Walter Bartram's son, Alan, said the family decided to place the heirloom "in safe hands."

"We've been long term supporters of the South Australian Museum and it seems fitting that it should be passed onto the people of South Australia to enjoy," he said in a statement.































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The South Australian Museum purchased the opal with an Australian government grant. The Fire of Australia will be on show in the museum's foyer until February 28, before taking its place alongside the museum's extensive permanent collection of Australian precious opal.

Polished history

Although impressive in its own right, the Fire of Australia is not the most valuable opal in existence.

The largest and most valuable single piece of rough gem opal is the Olympic Australis , a 3.45-kilogram, 17,000-carat stone estimated to be worth about $1.9 million.

The stone was purchased by Altmann + Cherny jewelers in 1956, and has been on display in their showroom in Sydney, Australia, ever since, according to Fiona Altmann.