Rare red diamonds offered in Rio Tinto auction

LONDON, England - Rio Tinto is now offering four rare red diamonds, worth millions of dollars, among 55 stones from its Australian Argyle mine in an auction, pitting jewellers against ultra-wealthy investors and institutions.

Fancy gem-quality natural colour diamonds have increased in value in recent years due to their small numbers, growing demand and vigorous marketing.

The star lot in the auction is the "Argyle Cardinal", a 1.21 carat radiant-cut, fancy red diamond, which one trade source said could command over $3 million.

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"Red diamonds are rare and getting rarer," Alisa Moussaieff, co-owner of Moussaieff Jewellers, who has bought some of the world's most exceptional colour diamonds, told Reuters.

"Red diamond prices can go higher because there are so few of them about."

One of the rarest is the 5.1-carat Moussaieff Red, discovered by a Brazilian farmer and now owned by Moussaieff Jewellers. The stone is estimated by trade professionals to be worth well in excess of $25 million.

The diamonds are being shown in Sydney, New York, Hong Kong and Perth, where interested parties can examine them and make sealed bids. The auction closes on Oct. 8.

Analysts said the pricing of rare collectible assets such as red diamonds is difficult and the market illiquid.

But the limited number of mines and exploration opportunities and the rising demand in markets such as China mean that prospects are good for gem-quality diamonds to rise in value, Marc Elliot, a diamond analyst with Investec, said.

Paula Vance, CEO of Swiss-based DeVanx Assets, which advises wealthy individuals and institutions on investing in red diamonds and hosts a trading platform, said value is based on factors such as hue, the cost of finding and extracting a red diamond and the numbers of such stones that come to the market.

The remainder of the stones in the Rio Tinto auction are pink and purplish red diamonds, which also attract strong demand.