Rio Tinto has unveiled a record four extremely rare "red" diamonds in an exclusive annual sale, with the miner saying there was growing interest in the jewels as collectibles and alternative investments.
The diamond tender, with stones sourced from Rio's Argyle mine in Western Australia, will be comprised of 55 diamonds, including 51 pink and purplish reds and four fancy reds.
The "hero" jewel at the centre of the sale -- in its 30th year -- is the Argyle Cardinal, a 1.21 carat radiant cut fancy red named after a small, northern American bird.
The jewels routinely fetch US$1-2 million a carat. As a basic rule of thumb, pink and red diamonds are worth about 50 times more than white diamonds.
In the three-decade history of the yearly sale, only 13 fancy reds have been put on tender and there are fewer than 30 graded by the Gemological Institute of America in the world, Josephine Johnson, manager of Rio's Argyle Pink Diamonds division, told AFP.
"Our traditional markets -- the heartland of the business -- is really in Australia, Japan, United States and Europe," she said.
"However, in the last three to four years, we've seen the emergence of particularly India and China, and now we see that the around 50 to 65 diamonds each year really go quite equally to all corners of the globe."
Johnson said there was also increasing interest in the fancy-coloured diamond market from financial institutions.
"Five years ago, I would have said that maybe 10 to 20 percent of our diamonds went into collections -- never left the box, held in a vault somewhere," she said.
"Now it's more like half, and the level of enquiry that we get around our tender-quality diamonds from banks and investment houses has certainly increased over the last two years."
After Sydney, the diamonds will be viewed by clients and experts in New York, Hong Kong and Perth. Bidding in the sale closes on October 8.
The largest red to come from the mine, the 1.56 carat Argyle Phoenix, sold for more than US$2 million to a Singapore-based jeweller in October.
It was the highest per-carat price paid for any diamond from the mine at a tender.
Another record was set at the tender when the Argyle Dauphine, a 2.51 carat Fancy Deep Pink diamond, sold for more than US$2 million to a US-based dealer.
It is not known how the diamonds acquire their pink or red tinge but it is thought to come from a molecular structure distortion as the jewel forms in the earth's crust or makes its way to the surface.
The Argyle mine produces more than 90 percent of the world's pink diamonds.