Geneva - Two rare coloured diamonds go under the auction hammer this week in Geneva, with one standout blue diamond discovered in a South African mine last year expected to fetch up to R786m ($55m) - which would set a world record for any gemstone.
On Tuesday, Christie's starts off the two-day blitz of jewellery sales with an auction headlined by "The Pink," a 16.08-carat diamond billed as rare for pure color. Its estimated pre-sale price is between 21 million and 27 million Swiss francs ($23-28m).
Sotheby's expects to fetch $35-$55m a day later with the sale of the 12.03-carat Blue Moon Diamond, said to be among the largest known fancy vivid blue diamonds.
The rare blue diamonds are formed when boron is mixed with carbon when the gem is created.
At that top price, the Blue Moon - so-called in reference to its rarity, playing off the expression "once in a blue moon" - would eclipse the record for a diamond at auction: The Graff Pink sold for $46 158 674 at Sotheby's Geneva five years ago, the auction house said.
The polished blue gem was cut from a 29.6-carat diamond discovered last year in South Africa's Cullinan mine, which also yielded the 530-carat Star of Africa blue diamond that's part of the British crown jewels and the Smithsonian Institution's "Blue Heart," discovered in 1908.
David Bennett, chairperson of Sotheby's international jewellery division, predicted the Blue Moon "will now take its place among the most famous gems in the world".
The diamond market could use a boost.
Spokesperson Margaux Donckier of the Antwerp World Diamond Center said slower economic growth in China and elsewhere has softened the overall diamond market, but auctions of rare quality pieces was likely to defy market forces.
"These are exceptional stones and will always be worth a lot of money," she said.