An extremely rare 37.3-carat pink diamond goes under the hammer Wednesday as the top offer at Sotheby's jewel sale, after rival auction house Christie's sold a whopping gem for nearly $34 million.

The twice-yearly precious jewel sales held at glitzy hotels overlooking lake Geneva have been raking in eye-watering sums in recent years and the demand for coloured diamonds remains strong.

Sotheby's has put a target price of $30 million (25 million euros) on the "Raj Pink", which was discovered as a rough crystal in South Africa in 2015, before being refined by a master cutter.

That price would be far short of the record $71.2 million (60 million euros) that the 59.6-carat "Pink Star" earned in April, the highest amount ever paid for any gemstone at auction.

But the Raj Pink will still be a unique acquisition for Wednesday's successful buyer, according to the Gemological Institute of America, which certified the diamond as a "Fancy Intense Pink", meaning it has unusually clear and natural colour.

"Only on the rarest occasions do diamonds with vibrant, shocking colour like the Raj Pink's emerge", the GIA said.

The top price ever earned at auction in the Fancy Intense Pink category came in 2010, when "The Graff Pink" went for a $46.2 million.

Sotheby's will be looking to beat expectations for the Raj Pink, which Christie's failed to do at its sale in Geneva on Tuesday.

The 163.41-carat and completely colourless "Art of Grisogono" sold for 33.5 million Swiss francs ($33.8 million), after taxes and commissions.

Christie's labelled that a world record for a gem of its kind, meaning it was the highest price ever earned at auction for a so-called D colour diamond.

But an industry expert said the price paid by the anonymous buyer fell short of expectations and may be a warning sign of a softening diamond market.

"I am disappointed that the Art of de Grisogono didn't sell for a more dazzling price" said Tobias Kormind, head of 77 Diamonds, a major European online diamond jeweller.

"This is a worrying sign for the top end of the diamond market suggesting it is not as immune as one would have hoped to downward price pressures caused by global factors such as the Chinese slowdown, Russian sanctions and lower oil prices," he added in a statement.

Source: AFP