Geneva (AFP) - A 12.03-carat blue diamond could fetch a record US$55 million (S$78 million) when it goes under the hammer at Sotheby's auction house in Geneva on Wednesday (Nov 11).

The "fancy vivid blue diamond" Blue Moon, discovered in South Africa in January last year, is the largest cushion-shaped stone in that category to ever appear at auction.

Sotheby's put its estimated sale price between US$35 million to US$55 million which, at the higher end, would mark an all-time record for the sale of a diamond at auction.

"Fancy vivid" is the highest category for coloured diamonds.

On the eve of that event, Sotheby's competitor Christie's is set to auction off a cushion-shaped 16.09-carat pink diamond that is tipped to fetch up to US$28 million.

Christie's said only three fancy vivid pink diamonds of over 10 carats have been up for sale in 250 years.

The flawless pink diamond is the largest in its class ever graded by the Gemological Institute of America (GIA). The sparkling oval-cut rock has received the highest possible GIA colour and clarity rating.

The world record for a diamond sale at auction is currently held by a 24.78-carat pink diamond, known as the Graff Pink, which sold in Geneva in November 2010 for just over US$46 million.

In November 2013 a plum-sized diamond that weighed in at 59.60-carats and was known as the "Pink Star", was auctioned by Sotheby's in Geneva for US$83 million, a world record for a gem, but the buyer, New York diamond cutter Isaac Wolf, defaulted on the payment.

The current record for a blue diamond belongs to the Zoe Diamond, which in November 2014 fetched US$32.6 million in New York.

The stunning 9.75 carat, pear-shaped diamond was bought by a private Hong Kong collector.

The price also set a new world auction record for price-per-carat for any diamond, the auctioneers Sotheby's said.

Only five white diamonds weighing more than 100 carats have been sold at auction, according to Sotheby's.

The most expensive was the 118.28-carat gem, which sold by Sotheby's for US$30.6 million at a Hong Kong auction in October 2013.

In May 2014, a 100.09-carat yellow diamond sold for US$16.3 million at auction in Geneva, a record for a diamond of this colour.

The Graff Vivid Yellow Diamond is one of the largest yellow diamonds in the world and its colour is described as "daffodil yellow".