A rare 74.48ct fancy yellow diamond named after the Greek god of the sun Helios will headline the tender.

The major's GM for sales and marketing, Patrick Coppens, said the Diavik Helios stone was a scarce find from the Diavik mine in Canada's Northwest Territories, which mainly produces high-quality white diamonds. Since production started in 2003, it has produced on average five large yellow diamonds per year, accounting for less than 0.001% of the mine's annual output.

Coppens expects the stone to be in strong demand from coloured diamond specialists globally.

Rio said the tender would also feature the last specials from the soon-to-close Argyle mine in Western Australia. It will be the last chance to buy 28,399ct of rough Argyle diamonds, sought after for their vivid colours. The tender will also feature a 26ct white gem-quality stone from the mine.

Since production started at Argyle in 1983, it had produced 865Mct of rough diamonds, distinguishing itself as one of the world's all-time great diamond mines.

Bids will close on November 9.

The way forward

Rio's diamond business unit, one of the largest producers in the world, is losing steam with Argyle falling away, and Diavik having only 3-5 years of production left.

Rio chief J-S Jacques has previously stated the company wants to remain in the diamond business and its exploration spend reflected that.

Last year the company spent US$523 million, an increase of 28% yoy, on exploration and evaluation of its highest-value projects in the Copper & Diamonds segment, which includes the FalCon project in Saskatchewan, Canada. A 2018 study on the project had suggested a 38-year mine recovering 66Mct of diamonds, with initial capex of C$1.4 billion, an after-tax NPV7 of $2 billion and an IRR of 19%.

However, the project is mired in a legal dispute with junior Star Diamond Corp which had accused Rio in March of non-compliance under its option to form a joint venture on the project. A circa $70 million, 2017 agreement had granted Rio four options to acquire up to 60% of the project.

The Court of Queen's Bench for Saskatchewan in August agreed Star had established a "serious issue" to be tried regarding its key claims against Rio Tinto Exploration Canada. The case continues.

Meanwhile, Star continues to de-risk the project, reporting last week a fourth bulk sample had returned 3,005 diamonds weighing 131.82ct, including 2.69ct, 20.03ct and 1.26ct stones.

Star took the opportunity to accuse Rio of employing "unproven" trenching technologies to bulk sample the kimberlite, causing undue breakage. It believed the method resulted in "significant unnecessary cost overruns" and was materially damaging Star's interest in the project. It seeks to remove Rio from the project "if necessary".