Global miner Rio Tinto said on Monday there was unprecedented demand for its rare pink and blue diamonds at its annual sale, with India and Japan among the keen buyers.

The 2012 Argyle Pink Diamonds Tender, seen as the world's most exclusive diamond sale, featured 56 single pink diamonds, including two red stones, and 19 blue diamonds from the company's Australian mine.

Rio said there was incredible demand for the jewels, which routinely fetch US$1 million a carat. As a basic rule of thumb, a pink diamond is worth about 50 times more than a white diamond.

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

Most successful bidders were from established markets such as Australia, Europe, Japan and the United States, as well as emerging customers like China and India, which bought several diamonds.

The top blue diamond, the Argyle Elektra, went to Japan, the world's most established rare diamond market.

Individual prices were not revealed. But the most valuable piece, the Argyle Siren, a 1.32 carat square radiant cut purplish pink diamond, was expected to sell for more than US$2.5 million when made into jewellery.

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

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

The Argyle mine in Western Australia produces virtually the entire world's supply of rare pink diamonds, with the red seen as the pinnacle of the colour scale.

In more than 26 years of tenders, Rio has only had 33 red diamonds.

It is not known how the diamonds acquire their pink 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.

[Image via Agence France-Presse]