Rio Tinto's rare pink and blue diamonds has attracted an unprecedented number of investor interest, particularly from Asian and European buyers, despite the world being in a massive fiscal crisis.

The global miner said on Monday that at the 2012 Argyle Pink Diamonds Tender, which featured 56 single pink diamonds, including two red stones, as well as 19 blue diamonds from the company's Australian mine, it was noticed that the successful bidders came from both established and emerging markets, noting that each carat already fetched US$1 million.

"We have experienced extraordinary and unprecedented demand reflected in number of bidders, prices and the final result of the 2012 Argyle Pink Diamonds Tender," Josephine Johnson, Argyle Pink Diamonds manager, told the AFP.

"There was very strong bidding - we have never had so many people disappointed. By way of example, one customer bid strongly on every single diamond and was unsuccessful."

The bidding process, which has always been kept confidential, received bidders from Australia, Europe, Japan and the United States, as well as China and India. Rio Tinto hinted that the 1.32 carat square radiant cut purplish pink diamond called Argyle Siren, the most valuable in the collection, could sell more than US$2.5 million when made into jewellery.

Most of the diamonds are headed for India, while the 'her' blue diamond, called Argyle Elektra, is bound for Japan.

"We are close to a tipping point regarding price, demand and global awareness of Argyle pink diamonds," Ms Johnson said.

"There is a near-obsessive quest for this ultimate natural rarity."

Rio Tinto's Argyle mine gives off more than 90 per cent of the world's pink diamonds.

"These rare Argyle diamonds provide the escalating intrinsic value of an investment and are destined for the exclusive echelons of the world's most sophisticated collectors," said Jean-Marc Lieberherr, chief commercial officer of Rio Tinto Diamonds.