This rare sparkler may be worth millions but its discovery at Argyle in the Kimberley won’t be enough to stop the mine’s closure next year.

Mine operator Rio Tinto says the Argyle Octavia is one of the biggest gem-quality white diamonds found at the mine over its 35-year history.

The 28.84-carat rock was recovered from the east Kimberley mine last month.

Argyle Diamonds General Manager Andrew Wilson said the Argyle Octavia was unique in its size, shape and provenance and would take its place in history as one of a few special large white diamonds ever to be produced from the mine.

The stone will be sold by private tender at the world’s diamond capital, Antwerp in Belgium, later this year.

Collectors, jewellers and traders will be invited to place confidential bids, with the rough diamond going to the highest bidder.

The rock is expected to attract strong bidding given only about 20 similar stones have been recovered from Argyle and the mine’s imminent closure means it will be considered a legacy piece.

Its rare and beautiful octahedral shape is also likely to attract a strong premium compared to similar gems.

Alluvial diamond mining started at Argyle in 1983, moving to an open pit operation between 1985 and 2013, followed by an underground operation from 2013 using a block cave method of mining.

But there is no economic case for continuing mining at even deeper levels past 2020.

Rio will seek to redeploy the more than 450 staff and contractors at the mine to its other operations as the mine winds down.

Some staff will remain to help with the rehabilitation of the site, which is expected to cost hundreds of millions of dollar over several decades.