Last Argyle pink diamonds are about to be sold at auction by Rio Tinto

Travelling more than 13,000 kilometres to the richest-known source of pink diamonds in the world wasn't easy.

But, for Danish jeweller Ulrik Hartmann, reaching Rio Tinto's Argyle diamond mine in the remote Kimberley region of Western Australia was worth every hour of his two-day journey halfway around the world.

"It was a dream come true," Mr Hartmann said.

"This very rough outback in the East Kimberley. It's just surprising to see where they actually produce some of the most fabulous diamonds in the world and to see the people who were mining the diamonds was amazing."

Mr Hartmann is one of only about 100 jewellers and diamond traders who are invited to bid for the prized diamonds at the annual Argyle Pink Diamonds Tender.

After 38 years, the Argyle mine was closed last November and the final sale is about to take place, to offload the last of the signature pink diamonds to the highest bidders in a silent auction.

"When we go to the Argyle Pink Diamonds Tender, it's like Christmas Eve for us," Mr Hartmann explained.

"I was surprised to see the sizes that were among the tender, 60 per cent of the stones are larger than one carat, which is a very rare size in Argyle."

The 70 diamonds up for sale, the most ever in the annual tender, range from the softest of pinks to the brightest of reds.

They've been travelling around the world with Michelle Sherring, Rio Tinto's manager of sales and business development.

"At the very pinnacle are our five hero diamonds," Ms Sherring said.

They're headlined by Lot One, the Argyle Eclipse.

The 3.47-carat Fancy Intense Pink, radiant-cut diamond is the largest of its kind to ever be presented for sale.

Of course, COVID-19 has meant the usual multi-country, multi-city tour was restricted this year. For example, none of those bidding in Sydney were able to see the diamonds in person.

"The Argyle tender is the most highly anticipated diamond event in the world," Ms Sherring told The Business.

"To present the stones to those who have long participated in the tender was a real delight."

Mr Hartmann's recent viewing of the stones in Europe was a bittersweet moment.

"It was a little bit sad because I've been going to the tender for more than 15 years and I will miss that special occasion where you sit with literally one year's special harvest of the finest stones in front of you," Mr Hartmann told The Business.

"There's no other mine in the world that produces colours like Argyle, from the red diamond to the more lighter pink shade."

Why all the secrecy?

The annual tender is enveloped in mystery.

A jeweller, gemologist or diamond dealer can only bid by invitation.

To be invited, they need to prove they're in it for the long haul, will promote the brand and, of course, show they have the cash available to pay.

Only those invited to bid can view the diamonds, and most of the time that's only through the specially made box they're held in.

The viewings are always in a secret location and the bidders don't find out where they're going until the very last minute.

Bids are made via a secure online portal and, once the tender closes, the winning bidder should expect a phone call within hours.

"After you win the bid, you have just four days to pay," explained David Fardon, a pink diamond specialist at Perth jeweller, Linneys. "And there's no backing out."

The winning bids are never made public.

While Mr Fardon is on the bidder's side of the deal these days, for about 15 years he worked for Argyle, managing the global tender.

"On average, they're 35 times the price of a white diamond, so they really are in a league of their own," Mr Fardon told The Business.

Now that the last of the large Argyle pink diamonds are about to be sold, many expect those prices to soar even higher.

Since 2000, the prices of the pinks sold at the annual tender have increased by 600 per cent.

That's growth the Dow Jones, Hang Seng or the ASX share indices could only dream of.

The final tender will also include 24 carats of blue and violet diamonds from the Argyle mine … they're even rarer than the pinks.

"It's taken us the course of many years to be able to accumulate enough to be able to put together a beautiful curation of blue and violet diamonds," Ms Sherring said.

Two champagne glasses full

Of all the diamonds unearthed at the remote Kimberley site during its almost 40-year mine life, less than 1 per cent were pink.

"If you take the 865 million carats that have been mined, about 1,900 stones a year would be pink," explained Rio Tinto's chief executive of minerals, Sinead Kaufman.

Just two glasses, from enough ore dug out of the ground to fill 38,000 Olympic-sized swimming pools.

Sparkling termite mound

Argyle accounts for about 90 per cent of all the pink diamonds that have been found in the world so far.

Finding them was not an easy task.

During the 1930's, University of Western Australia's professor of geology Rex Prider wrote that parts of the Kimberley's geography looked a lot like the land where diamonds were mined in South Africa.

Decades later, Ewen Tyler — a director for British miner, Tanganyika Holdings — decided to test Professor Prider's theory.

In 1970, he launched a $100,000 expedition with four other companies, including miners and a diamond producer, to search for the sparkling gems.

The brief was not easy: To search 225,000 square kilometres of rugged land, an area just smaller than the entire United Kingdom, for diamonds buried in the iconic red earth.

It took four years to find the first specks of diamond and indicator minerals.

However, it was not until October 2, 1979, that the big discovery was made.

After previously finding diamonds and minerals in old riverbeds — known as alluvial diamonds — the search party was looking in earnest for the old volcano pipe that brought the diamonds up from deep within the earth more than a million years ago.

"That gave a good indication they were close to the pipe," explained Ms Kaufman.

It turned out the geologists were standing on top of what became known as the AK1 pipe.

That pipe became the largest diamond mine in the world and, in just its first few years of operation, doubled the number of known diamonds in existence.

Over the years, ownership of the mine has changed hands, but for more than half of its existence it was owned by a conglomerate.

One of its earliest members, since 1976, was CRA Exploration, which later became Rio Tinto and, ultimately, the sole owner of the Argyle diamond mine in 2002.

Changing way diamonds are sold

In the early years, global diamond trader De Beers was enlisted — as it was by most diamond mines at the time — to manage the sale of the gems.

De Beers was mostly interested in white, or colourless, diamonds, practically dismissing the brown, or champagne, diamonds that had been unearthed, as well as the pinks.

That set Argyle off on its own path to market and promote the coloured diamonds as more prized than the colourless stones.

It broke new territory by doing so and, in 1985, opened a central office in Antwerp, the diamond-trading headquarters of the world.

It was not long before Argyle diamonds started to arrive in the Belgian city by the truckload.

Argyle began its pink diamond tender in Antwerp that same year, selling to select jewellers and ateliers, as well as marketing its champagne diamonds.

By the mid-1990s, Argyle ended its relationship with De Beers and began selling all of its diamonds itself.

At that point, about 42 million carats were being mined each year.

Last of the pinks

The winning bids for the final tender, and a lasting piece of Australian gemological history, will be awarded on October 5.

It will be the 30th time that Linneys will bid for the coveted pinks.

Creative director Justin Linney said working with the diamonds was unlike working with any other stone.

"With pink diamonds, because of their rarity, our design process has to be reversed," he said.

"We have to work with the pinks first, establish what we have available, then design the pieces around the pinks.

"Whereas, if it's with a white diamond, we design the piece first, then find the diamond."

The jeweller has perhaps one of the most unique pieces ever created using Argyle pink diamonds: A tiara worth more than $5 million.

Lady Gaga almost bought it once, but it is still on the market.

Both Linneys Jewellers and Ulrik Hartmann will be waiting by the phone next week, to see if their bids to snap up a final piece of history are successful.

"If you want to be a part of the Argyle club, it's still possible to buy a piece of jewellery," Mr Hartmann said.

Perhaps, but just not for much longer.