An Australian gemstone, billed as one of the largest gem-quality opals ever found, has sold for $US143,750 ($A200,000) at auction in Alaska.

The opal, dubbed the “Americus Australis”, weighs more than 11,800 carats. It also has a long history.

Most recently, it was kept in a linen closet in a home in Big Lake, north of Anchorage, by Fred von Brandt, who mines for gold in Alaska and whose family has deep roots in the gem and rock business.

The opal is larger than a brick and is broken into two pieces, which von Brandt said was a practice used decades ago to prove gem quality.

Von Brandt said the stone has been in his family since the late 1950s, when his grandfather bought it from an Australian opal dealer named John Altmann.

Von Brandt said the opal for decades was in the care of his father, Guy von Brandt, who decided it had been “locked up long enough, that it’s time to put it back out in the world and see what interest it can generate”.

“He entrusted me to figure out which direction we wanted to go to part with the stone,” von Brandt said prior to the sale on Sunday.

The family, with roots in California, exhibited the stone at gem shows for years, until the early 1980s. His father then branched out into furniture and displayed it at his shop.

Guy von Brandt eventually moved to Oregon and kept the stone “kind of tucked away” for many years, von Brandt said.

Von Brandt said he brought it with him to Alaska over a year ago as he weighed the best approach to a possible sale.

Nick Cline, a partner and appraisal specialist with Alaska Premier Auctions & Appraisals, said the family had documentation surrounding the provenance of the opal.

As part of his research, he contacted Fiona Altmann, granddaughter of John Altmann and general manager of Altmann + Cherny in Sydney, Australia.

Altmann said her grandfather, in his business dealings, made regular trips to Europe and the US.

Altmann said when Cline emailed her, she was skeptical; the name of the stone, in particular, threw her. But she said she started digging and discovered “something with my grandfather’s handwriting with the picture of the opal with the word ‘Americus Australis’.”

“I with 100 per cent certainty know that their provenance information is 100 per cent accurate” because it lines up with information she has, she said.

The auction house said the stone was discovered in the same field in Australia as the opal known as the “Olympic Australis”, which weighs 17,000 carats and is on permanent display in Altmann’s shop. 

The Olympic had been among the stones that John Altmann and partner Rudi Cherny acquired in 1956, according to Altmann’s company.

AAP

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