The world's finest uncut opal, the Fire of Australia, has become part of the South Australian Museum's collection more than 60 years after it was discovered.

The 998-gram opal valued at nearly $900,000 has been on loan to the institution but will become part of the collection after a $455,000 grant from the Federal Government and the Bartram family.

Alan Bartram said his father, Walter, mined the Fire of Australia in 1946 at the Eight Mile field in Coober Pedy, South Australia.

Mr Bartram said his family, which has been in the opal industry for 80 years, was one of many who mined opals at the "prolific" Eight Mile field.

"At the end of the war all of the sons and siblings and greater families were all invited to come and join in this prolific field, which was absolutely exceptional and not very deep so they could do it with hand mining," he said.

"Everybody that was there was successful, some to huge extent.

"It was just one of those brilliant experiences to be able to be involved in the Eight Mile at Coober Pedy at that time."

Mr Bartram said while he could have fetched a much higher price for the Fire of Australia at international auction, it was important to him that it remained in South Australia.

"It is such a piece, so outstanding that it would have been a sheer misery to see it go to another destination and be cut up for watch faces or something like that," he said.

"After loaning the Fire of Australia to the South Australian Museum for its Opals exhibition, we made the decision to place this family heirloom in safe hands."

The exhibition was the most visited in the museum's history.

Jane Lomax-Smith with Senator Mitch Fifield and Alan Bartram (right) with Fire of Australia opal, which weights almost 1 kilogram. ( ABC News: Caroline Winter )

Mr Bartram said the opal was made up of almost 5,000 carats — and can be compared in size to two cricket balls, side-by-side.

"I have never seen anything of the size and the quality," he said.

"Carats are a pretty small item, but 5,000 of them amounts to quite a substantial piece. Just outstanding, [an] exquisite piece in all ways."

The opal spent many years at home with Mr Bartram's father.

But given its value, Mr Bartram was also sure to keep it in a safe place over the years.

"Given that this was such an important piece, it spent time when my father had it at home, and it was out of the way I can tell you," he said.

"But I also introduced it to the safe deposit area of the Commonwealth Bank. It spent quite a few years there and I would visit and make sure that it was safe and it was not deteriorating in any way.

"It has spent time elsewhere in many exhibitions here and overseas."

'99pc of world's most precious opals found in SA'

Museum director Brian Oldman said the opal was rare.

"Opal of this quality can only be created under certain climate conditions," Mr Oldman said.

"Ninety per cent of the world's most precious opals are found in South Australia.

"When our state's inland sea evaporated millions of years ago it provided a unique silica-rich environment for the creation of precious opal."

The Fire of Australia will be on display in the museum's front foyer until February 28.