The Rock, the biggest white diamond ever auctioned, sold for a hammer price of 18.6 million Swiss francs ($18.8 million) on Wednesday, far short of the record for such a jewel.

The 228.31-carat stone, larger than a golf ball, was sold in Geneva by Christie's auction house.

There had been high hopes that The Rock would smash the world record for a white diamond, which stands at at $33.7 million, fetched in the Swiss city in 2017 for a 163.41-carat gem.

The bidding, which started at 14 million francs, came to a halt after two minutes at 18.6 million, though the price will increase once taxes and the buyer's premium are added on.

The pre-sale estimate had been 19 to 30 million Swiss francs.

The Rock, a perfectly symmetrical pear-shaped diamond, was in the hands of an unnamed owner from North America. It was bought by a telephone bidder following the action at the Hotel des Bergues.

Max Fawcett, head of the jewels department at Christie's auction house in Geneva, said there were only a handful of diamonds of similar size and quality to The Rock.

The large diamond was extracted from a mine in South Africa in the early 2000s and has been shown in Dubai, Taipei and New York ahead of the sale in Geneva.

Red Cross gem

On sale later in the Magnificent Jewel auction is an historic intense yellow diamond associated for more than a century with the Red Cross.

The Red Cross Diamond is a cushion-shaped, 205.07-carat canary yellow jewel, which has a price estimate of seven to 10 million Swiss francs ($7.09 to $10.13 million).

A large chunk of the proceeds will be donated to the International Committee of the Red Cross, which is headquartered in Geneva.

The original rough stone was found in 1901 in a De Beers company mine in South Africa and is said to have weighed around 375 carats.

As well as ranking among the largest diamonds in the world, a striking feature is its pavilion, which naturally bears the shape of a Maltese cross.

The stone was first put up for sale on April 10, 1918 at Christie's in London. It was offered by the Diamond Syndicate in aid of the British Red Cross Society and the Order of St John.

The Red Cross Diamond fetched £10,000 -- approximately £600,000 ($740,000) in today's money. It was bought by the London jewellers S.J. Phillips.

It was sold again by Christie's in Geneva in 1973, fetching 1.8 million Swiss francs, and is now being offered by the auction house for a third time.

Also being sold is a tiara that belonged to Princess Irma of Furstenberg (1867-1948), a member of one of the most pre-eminent aristocratic families in the Habsburg Empire.

It is estimated to go for 400,000 to 600,000 Swiss francs.